Wheels properly aligned are parallel to each other and, at the same time, perpendicular to the ground. Wheel alignment is not to be confused with wheel balancing. If the wheels are out of balance, instead of causing a pull on the steering wheel, it will likely cause vibrations at higher speeds, such as on the freeway, as the wheels wobble. This will more likely cause the steering wheel to wiggle or vibrate, rather than pull steadily to one side.
One aspect of wheel alignment is the camber. The camber is the angle of the wheel when looked at from the front of the vehicle. When the top of a tire is leaning out away from the car, it is referred to as a positive camber, meaning too far out. If it is leaning in towards the center of the car, it is called a negative chamber, meaning too far in. Positive and negative cambers can both cause premature wear on one side of the tire. With positive cambers, the tire wears quickly on the outside of the tread. But a negative camber may be harder for the layperson to spot, as the wear happens on the inside tire tread.
A camber that is off long enough will cause the tires to wear unevenly and will cause the steering wheel to pull to one side. Whichever side has more positive camber, the steering wheel will lean that way, especially as the tires wear down on that side. But not all vehicles have adjustable cambers. Sometimes they need repair or replacement due to damage caused by an accident.
Another term in wheel alignment is the caster. The caster is the angle of the steering pivot, which is what turns the wheels when you pull your steering wheel to the right of left. If the pivot is leaning toward the back of the car, it is positive. It is negative if the pivot is leaning toward the front. When the caster is maladjusted, the wheels will pull to the side with the more positive castor. If both sides are equal, but too negative, it makes it hard to keep the car in a straight line, either way. Overly positive caster adjustment can cause the steering wheel to push when the car goes over a bump. The caster is also not adjustable and is only typically off if there has been an accident.
The toe-in is the comparison between the distance between the front of the two front tires to the distance between the back of the two front tires. In other words, a mechanic will measure from the center front of the left side front tire to the center front of the right side tire. Then the same is done to the back of the front tires. If the distances are matched, then the tires are properly parallel. If not, then the tires will wear rapidly on both sides.
Proper wheel alignment may be an unwanted expense, but it is certainly a vital one. Those who drive around with improper alignment will spend more in tires than they would on getting the alignment done. For that reason, it is recommended that it be checked whenever there is reason to suspect that it is off. It is also a good idea to have it checked when you are getting ready for a long drive (read more tips for tuning up for a long drive).