A Wheel alignment is the adjusting of the angles of your car's wheels relative to the ground to achieve maximum tire life. A properly aligned vehicle with good tires should drive straight along a level, straight road.
An alignment is recommended once a year for a couple of reasons:
1. There are various moving joints and rubber bushings in your vehicle's suspension system that wears very slowly over time. As these parts wear, the wheel's angles change and need to be re-adjusted to compensate. These parts should be routinely inspected and replaced if they are worn beyond a normal threshold.
2. Running through a good size pot hole or even hitting a curb can very well knock your car's alignment out of whack - ouch! I think we're all guilty of that!
It is also recommended to align your car when you install new tires; that way your new tires get a fresh start for the longest life possible!