Mobs have threat lists, also known as hate lists. This is a list of all the characters that have angered the mob, ordered from the one with the largest amount of threat down to the least. Generally speaking, if a mob kills the player with the most threat (at the top of the list), it will then move to attack the next on the list.
The person the mob attacks is the person with aggro. Though in theory we would say that the player with the most threat will also have aggro, in practice it is a bit more fuzzy.
In groups, the actions each player takes with respect to the mob will move them up or down the threat list. However, the other members of the group are also moving up or down the threat list, so even if you perform no actions your relative position on the list may change.
Things that move the player up the threat list:
- Damage dealt
- Aggro-gaining skills and abilities such as Sunder Armor and Taunt
- Healing a player on the threat list
Things that move the player down the threat list:
- Druid Cower ability
- Rogue Feint ability
- Priest Fade ability
- Rogue Vanish ability
- Hunter Disengage ability
- Warlock Soulshatter ability
- Mage Invisibility ability (during the 5-second fading)
- Some Mobs spells or debuffs such as Onyxia Knock Away ability.
Things that remove the player from the threat list completely:
- Running far enough away (if not in an instance)
- Hunter Feign Death ability
- Mage Invisibility ability (after the 5-second fading)
- The Paladin ability Divine Intervention
- Some mob spells or debuffs such as Chromaguss's Time Lapse debuff