Monster Level
Every monster has a level, which you can check by select the monster then looking at its target window. Generally, it's a good idea to attack monsters below or near your level. If the monster's level is listed as "??" or a skull it indicates that you cannot determine the level of the monster, but it's considerably higher in level than you.

Elite Monsters
Elite monsters are much more powerful versions of their regular counterparts. They are typically either special quest monsters, or monsters that are found inside dungeons. They can be identified by an "Elite" tag next to their name and a special dragon border around their target window.

Monster Aggression
There are three main types of status for monsters:

Aggressive - Aggressive monsters will attack when you enter their aggro range. You can identify aggressive targets by the red color of their name box (target window).

Neutral - Neutral monsters, such as many wild animals, will not attack unless first attacked. You can identify neutral targets by the yellow color of their name box (target window).

Friendly - You cannot attack friendly targets. Examples of these are party members and NPCs of your faction. You can identify friendly targets by the green color of their name box (target window).

Aggro Radius
Each Aggressive Monster has an aggro radius, in an invisible circle around the monster. Entering a monster's aggro radius will provoke it to attack. The size of the monster's aggro radius is based on the level difference between you and the monster.

Solo vs. Social
Solo monsters will not help fellow monsters of the same type if they are attacked. For example, most boars don't care if you attack another boar. Social monsters, such as murlocs, will call for help. If any other monsters of the same allied network are around, they will assist each other.

Monster Hate List/History
Once combat has begun, monsters keep track of damage being done to them and also keep track of the spells being cast. This includes healing spells, or spells that buff other party members. Generally, the monster will attack the player who is currently highest on their hate list. This is fine if you want the monster to attack you; however if you're a caster you generally don't want this to happen. To counter this, another party member must do more damage to the monster or otherwise cause more threat so that the monster will attack the other party member instead. You will also need to immediately stop actions that will make the monster angry, such as casting more spells, or attacking the monster. Once another party member has done enough damage or made the monster angry enough to redirect its attention, you can resume your attack on that monster.

Monster Claiming
The first player or group to damage a monster will "tap" that monster. When the tapped monster dies, that player or group will get the experience and loot from the kill. Other players will be able to tell if a monster is tapped because its portrait will turn gray. Players are free to help kill the tapped monster, but they will receive no experience or loot from the kill. Even with help, the player or group who initially tapped the monster will gain full experience from the kill. In situations where a much higher level player helped with the kill, some of the experience reward will be reduced.

Fleeing from Monsters
You will likely be spending some time running away from monsters as you get accustomed to fighting in World of Warcraft. Running isn't something to be ashamed of: you should always run if you face a monster that is too powerful, or if you realize you don't have enough Health or Mana to fight the current monster. Run away! "/v flee"! Monsters will only chase you for a specific distance before giving up and returning to their original spot. In instances however, monsters will not stop their chase.

Monster Healing
Some monsters can heal themselves, just as some players can. You will need to interrupt their heals, or finish them off quickly with powerful attacks before they can heal. Typically, stun abilities are the best to use for this situation.

Monster Buffs and Debuffs
Like players, some monsters cast buffs on themselves to make themselves more powerful. You may discover this by watching them "buff up", or by reading the combat log and screen for results of the fight. Whenever possible, use a dispel ability on the monsters to remove their buffs. In addition, monsters often have special attacks which will place a "debuff" on your character.

Spellcasting Monsters
Spellcasting monsters, such as mages, are very dangerous as they can do a considerable amount of damage in a short time. Fortunately, they are typically weaker than monsters who rely on physical attacks and are thus more quickly dispatched. Attack them first, and try to stun them if possible to prevent them from casting spells.

Monster Fleeing
Some monsters will attempt to run away when they are near death, often in the direction of their comrades. You should use abilities that prevent them from moving or slow them down - or simply finish them off quickly as soon as they try to run.

Fighting High Level Monsters
Players will find it more difficult to fight monsters that are a good deal higher in level than they are. A player's chance to hit and damage ("glancing blows") is reduced, while the monster's chance to hit and damage ("crushing blows") is increased.

Monster Spawning
Monsters will respawn a certain amount of time after being slain. The time to respawn varies widely, from a few minutes in highly popular areas to hours in dungeons and raids. In lower level areas, monsters will not immediately attack players as they spawn.

Evading
Monsters "evade" when they are stuck or when they cannot reach their target. When evading, they cannot be attacked and will quickly regenerate health and mana.