Auto-aim

Auto-Aiming is a technique used by Bungie to make targeting your opponent easier. This shifts the challenge from aiming to movement and strategy, unlike most FPSs. Other games using Auto-Aim completely lock on to a target, while Halo simply makes it easier.

Face painting
Face painting is the most well known and used form of auto-aim. Utilizing this can give you a large advantage. It was implemented by Bungie to combat lag.

Assisted Aiming
There are two types of assisted aiming in Halo 2, Auto-Aim and Magnetism.

Auto-Aim defines how far a bullet will "bend" in order to hit an enemy. This is also affected by the speed at which the bullet moves.

Magnetism simply assists your aim by creating a sticky effect between your reticule and your target when in range. Unlike Halo 1, it will not work without you moving your reticle.