Auto-aim

Auto-Aiming is a technique used by Bungie in Halo 2 and Halo 2 Vista while using a controller, 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 counter combat lag.

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

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

Magnetism simply assists and advocates in your face painting and your aim by creating a sticky effect between your reticule and your target when in range. Unlike Halo: Combat Evolved, it will not work without you moving your reticle entirely.