The PvP experience will be changing drastically with the addition of PvP honor and rewards. This will change the behavior of many PvP players. Read more here.

Many players prefer the player vs. player realms for several reasons:
There is the excitement of facing real people in addition to computer-controlled monsters.
On normal realms, when a player of the opposite faction offends you, there is no real counter. On PvP servers, however, you can kill them!

Random tips

Here are some random general tips that new players might find useful while playing on a PVP server.

Understand the Region/Zone Types
If you're in a contested zone, you can be attacked by the enemy. If you're in a red (enemy) zone, they can attack you but you can't attack them until they strike first. If you're in a green (friendly) zone, they can't attack you unless you attack them first or you are flagged for PvP. Learn more.

Level Difference
It is possible to kill players much higher level than you. You can hit them. There are certain class match-ups that will work in your favor despite the level difference. It's much easier killing a higher-level player than a high-level monster. One of the main differences between different level characters is damage output. Thus, many lower-level players can take down a single higher-level player with their combined damage output.

Frequency of PvP Combat
The frequency with which hostile players of your level attack you decreases in higher-level regions. In low-level contested zones (level 20 to 30ish zones), players are usually new to PvPing and will want to attack any enemy they see just to experience PvP and without any apparent reason. In higher-level zones, players are used to it already, and PvP fights will usually take longer. It's harder to catch runners, so enemy players will often mind their own business and leave you alone. So don't get scared if you're getting killed too much in your first contested areas; it gets easier as you level up.

Classes
Try to learn how your class fares against other classes. That way you will know whether you should fight or flee when you spot the enemy. You can always try trial-and-error to begin with.

Races
Try to learn the enemies' racial abilities. Undead, for example, have an ability that helps counter Fear.

Pay Attention
Playing on a PvP server means paying attention. Look over your shoulder at almost all times. Know that your quest plans could change any moment and turn into a battle with the enemy faction. If you're not ready for this, consider a PVE server instead of or in addition to PVP. It's not for everyone, but you'll find that most people that play and enjoy a PVP server love the exhilaration of never being safe. It takes skill, it takes patience, and it requires that you keep up with strategies. It might mean more reading and studying than the PVE servers, but if you're reading this, chances are you're up for it. Good luck.

Dueling
Dueling can be a good way to test out your skills and equipment versus classes you may face in the opposite faction. Only the Shaman and Paladin are exclusive classes to each faction. It's also great to experiment with different combinations and new strategies.
Learn more.

Adventure in Groups
It's a lot safer to adventure with a full party. That way, if one or two enemy players attack, you'll have a better chance of surviving. Solo players are the most at risk and should be prepared to die more depending on their class, skill level, and items. A bunch of low-level players can take out a higher-level player with enough numbers. So if there is a high-level enemy killing many people in your region, rally defenders by using the LocalDefense channel and by using the /who command to check for other players around. Get together with other players who are getting killed by the same enemy you are, party with them, get yourselves organized together, and hunt him down.

Stay Together
Work with your group. Pay attention to who's attacking what target and stay together. Most groups are wiped out because they didn't work together on a target or because they spread out. Staying together, however, doesn't mean bunching together. Stay within range of helping each other. If you bunch, you'll be susceptible to area affects. It takes practice, but it's worth working on. Sometimes it's too easy to want to leave your group to chase down that almost dead enemy. Consider that you can be vulnerable as you leave your group and you can weaken them by leaving them a player short. Some teams actually work on this as a strategy, pulling apart strong groups or baiting them into ambushes. You're only as good as your weakest link.

Monitor the Channels
Pay attention to the general chat channel, LocalDefense, and WorldDefense. This can give you a heads up to enemy locations. If you want to help out, let people know in the general channel or in local defense, if you spot the enemy or see them attacking. Be sure to mention the enemy levels and class as others find this information very valuable.

Create a new chat window and set it to the WorldDefense. Whenever you're going to another area, check that chat window to see if it or any surrounding areas are under attack. If they are, you might want to go somewhere else, as you're likely to encounter enemy players.

Run Away vs. Stay and Fight
If you think you cannot win, run away. If you know you can't get away, you can just stand there and die. If you don't fight back at all, it takes some satisfaction away from the person killing you.

However, for other players, it may actually encourage them to attack you more because of your cowardice. You shouldn't bother running away if you know you can't escape. Some players recommend fighting bravely to your death instead of running away. There's no telling how the enemy player will react to your choice. It's a grab bag of possibilities.

It's almost always best to get the jump on the enemy. Every class in the game has an opener ability that will give you an advantage if you're the one who strikes first. Attacking first also gives you the element of surprise. So if you get jumped, sometimes you can choose to run away or "pretend" you're running, hide, sneak back, and jump on him.

Getting Away
There are several items that can really help you escape. Swiftness potions, created by alchemists, can increase your run speed for a short amount of time. This can allow you to get away. There are also other items that can temporarily boost your speed, such as the Nifty Stopwatch trinket.
You should also try to hide behind trees and whatever objects you can find. Remember, your name is above you in red which may give you away so you need to hide this as well.
Do not run in a straight path or where the enemy expects you to run. Take sudden turns and go in completely different directions. It's most effective to make a turn when the enemy loses sight of you (like in the movies). This can allow you to escape the enemy sometimes.
There are also other creative methods for getting away, such as a Druid running into water and escaping in aquatic form, a Priest fleeing on water with the Levitate spell, or a Mage jumping off a cliff with Slow Fall.
If the enemy is mounted and you are not, try to run into water, which will dismount them.
Try to run toward other members of your faction or your faction's local town if possible.
Hunters have tracking abilities which makes them harder to escape.

Stopping Players From Running
There are a lot of player spells that can prevent players from running: Polymorph, Fear, Howl of Terror, Charge, Stun spells, Entangling Roots, Concussive Shot, Frost Shock, Curse of Exhaustion, etc. Do some research on your class and see if any of those spells are available to you. Many spells used for catching players can also be used to run away.

Items
Gnomish Net-o-Matic Projectors, created and used by Gnomish Engineers are great to use in PVP. They have a long cooldown. There are a few one-time use items that can also stop players from running.

Stay off the Roads
If you want to avoid the enemy, stay off well-traveled roads. Take paths that players wouldn't normally take. You may still run into enemy players, but the chances are less. This is especially important in enemy territory.

Catching People in Water
Use Swim Speed potions. They increase your swimming to a very fast rate. You can also use an Azure Silk Belt, which also increases your swim speed. There are also the standard spells and items that can stop players from moving.

Chasing and Mounts
The level 60 mount is much faster than the level 40 mount. When chasing down other players, those with an epic mount have a huge PvP advantage over those that don't. Be aware of additional ways to increase mount speed. Mithril Spurs and a glove enchantment are both enhancements to your gear that will improve mounted speed. There is also a quest item called Carrot on a Stick that will grant the same benefit. These bonuses stack with each other. Having these speed boosts can allow you to catch or get away from a player with the same level of mount.

Use Terrain
Use the terrain to your advantage. Hide behind trees, logs, rocks, mountains, inside buildings, under bridges, or in the water. Hide there, bandage there, eat there, and be safe. It's not uncommon to see Hunters up on the hills or towers in big PVP battles.

Distances
Standing right next to that enemy warrior isn't such a good idea. Neither is running away and turning your back to that Hunter that can shoot you in the back from far away. Distance and direction are important in different ways for each class. Learn your ideal distances and learn to move. Many players from older MMOs are set in their ways and stand still to fight. It's a learned behavior that might get you killed. Though you might not be able to move while casting some spells or using some abilities, there are many instant or non-timer abilities that can be performed as you move and jump.

Always watch your health and mana
In heavily contested territories, you should be much more careful about your hit points and mana. There is no telling when an enemy might surprise you, so it's much better to always stay high on hit points and mana. Eat, drink, or bandage after every kill.

Stealth
Rogues and Druids are dangerous PvP opponents because of their stealth capabilities. You can often detect stealth by looking towards the stealthed character from a short distance or by listening for the distinctive "whoosh" sound that occurs when a character enters stealth mode or is detected in stealth mode. The higher level you are in relation to the stealthed character, the better your chance of detecting that character. Some skills such as Flare or Perception can remove stealth from nearby characters, and certain items, potions, and skills (Perception) can improve your chance of stealth detection. Faerie Fire, Hunter's Mark, and damage-over-time spells prevent a character from entering stealth mode.

If you are sure that there's a stealthed enemy nearby, then the easiest way to avoid them is to simply run away. Even at higher levels, the movement speed when stealthed is slower than normal.

Other Items
Catseye elixirs are useful for detecting stealthed Rogues.

Ganking
Ganking can mean being killed while fighting a monster or killing another player while they are fighting a monster. You WILL be ganked on a regular basis. Watch out for other players while fighting. Often, players will be consumed with their current fight and won't notice the enemy attacking from behind. Don't let your guard down.

Getting Your Body after Being Killed
There is always the possibility that the enemy is waiting for you at your corpse. Always prepare for this possibility. After returning to your body, scout the area and wait a bit before resurrecting. If you see the enemy, move as far away from them as possible, while still having the resurrect button active. If there are any buildings, trees, or objects you can hide behind when resurrecting, you should use them. But be warned that other players might think of that too.

If you have a mount, hit the resurrect button, then jam the mount summon button as soon as possible (it's recommended that you place it as a hotkey on your action bar). You can be mounted and running before the enemy has spotted you. At this point, you have a really good chance to get away.

Harassment
You may sometimes come across an enemy player that continually stalks you. Be aware of the PvP policy concerning such harassment here. You have several in-game options available to you for dealing with such a situation:
Continue to Fight - You can keep trying to get away and return to fight again.
Leave - go find another area or zone to play in. There are usually multiple options. Consult the region levels or quest levels page.
Get Help! - Use your guild or general chat channels to get reinforcements to kill the attacking enemy players. This can work sometimes but will not always work. You might have to deal with this situation on your own. If there are players in your party you can also organize them to defend yourself.

Killing Enemy Players
It's up to you to determine how far you want to push it in your war against enemy players. If it's a lopsided battle where you will easily defeat the enemy, you can choose not to attack them or just attack once and move on. You can certainly continue to fight the enemy should that be your choice. Keep in mind it's a game, and people should have fun on both sides. As long as you stay within the rules, you're OK.

Honor and Dishonor
It's important to remember that World of Warcraft is designed to be fun for both sides. If you take an action that could be considered unsportsmanlike, it impacts the enjoyment for the opposing side. The action may be reciprocated against you, and it will certainly enrage the opposing faction. Corpse camping (attempting to wait for a defeated foe to respawn so you can kill him again in his weakened resurrected state), killing players engaged with one or more NPCs, crushing players extremely far below your level, waiting by the graveyard to ambush enemies using the spirit healer, and using victory emotes excessively are examples of actions that may be considered dishonorable.

Sparing a weakened foe's life, declaring your antagonism before the battle, and using your emotes wisely may cause an enemy to honor and fear you without hating you as well.

However, you are at war with the opposing faction, and as they say, "All's fair in love and war." It is up to you to decide your own code of honor or dishonor. Perhaps as a sinister rogue, you prefer to cut them down by any means. Or maybe an honorable warrior prefers to grant his enemies a fair chance. The game itself will not penalize you for any poor sportsmanship short of exploiting the game mechanics for the sake of ruining another's fun.

Reputation
Be warned, constantly harassing and killing players over and over, attacking them when they are fighting monsters, and camping their corspes may hurt your reputation. Players may band together and hunt you down with extreme prejudice.

Celebrating your Kill
Many people use emotes on the enemy corpse to celebrate their kill. If the enemy player has not released their body then they can also see those. So, usually players hit release as soon as possible to avoid being "emoted on."

Item Durability
Getting killed by another player does not result in the normal loss in item durability as in death caused by a monster. If a player attacks you while you are engaged with a monster and you have to take a death, it is to your advantage to die to the player.

First Aid
First aid is a must, and as a secondary skill there is no reason you shouldn't develop it as fast as possible. Bandages can be essential in PvP combat, especially if you are playing with or against a class that has the ability to cast Fear, Polymorph, Root, and Stun spells or effects. You can use the quick timeout afforded by these spells to bandage. The higher your first aid skill, the better. Sometimes in the heat of battle you may not get the chance to complete your full bandage, so having a higher skill and better bandages will assure that you get more value from that time, no matter how many seconds you have. It can mean the difference between life and death, especially for low hit point classes. Remember too, bandages stack with potions.

Watching Enemy Casting Movements
You can sometimes tell when the enemy is casting a certain spell or a certain spell type, such as nature or shadow. You will then know how to counter it. On the other hand, the counter to that is to fake like you're casting one spell and then get the enemy to interrupt and switch to the REAL spell you want to cast (advanced users only).