Aggro is the system of mechanics which determines who enemies target, and why. If you know these mechanics, you can use them to your advantage.
For example, in WoW you can 'drop aggro' by reducing your dps, or by casting feign death, all of which adjust the formulas so that enemies will move you lower down on their 'list of things I want to kill'.
In DOS there are CLEARLY aggro mechanics, since one of the Talents (Stench) directly mentions them.
