Aggro and Threat

Every player has a threat rating for each mob thats in combat with them. As a player damages a mob their threat against that particular mob is increased. Other skills such as taunt generate threat in a special way.

Most monsters will attack the player that has the highest threat rating. If you have the highest threat and mobs start attacking you, then you are considered to have "aggro". This simply means that you are being attacked by something (or everything).

There is usually one player who is the designated tank. Their job isn't to do damage so much, its more about controlling the threat and holding on to aggro for as many mobs for as long as possible.

Reducing Threat

If you are not the tank and you want to reduce the threat there are a few things you can do.

1. Only attack the tank's target. You can assist the tank so you are only ever attacking their target. Be on your guard however as the tank can change targets in a large fight.
2. Delay your initial attack. A tank will try and build up threat on a mob or a group of mobs at the start of every combat. Depending on the class of the tank, they might want to build multiple Sunder Armor's (which increase threat), or they may simply be building up large amounts of damage.
3. Turn of your Pet's Taunt. Most pets will have a taunt ability which will try and gain enough threat from their target to gain aggro. When in groups however, this is usually not advised as a player tank is usually much better than a pet. Its common to disable the taunt ability of your pet while in a group with a designated tank.
4. Watch your aggro meter. There are various plugins that measure and display your threat rating in the party. The new 3.0 UI enhancements include something similar, albeit a bit simpler. You can use these graphs to watch your threat to make sure it doesn't go over the tanks.
5. Slow your Healing / DPS. Healing spells generate threat, so do DPS spells. If you delay your initial attack (or heal) to only when its needed you will reduce the chance that you will generate enough threat to gain aggro.