To get the best wear and performance from your tires, the alignment of the tires is critical. Poor alignment occurs when the suspension and steering systems are out of adjustment. Incorrect alignment settings will usually result in abnormal treadwear. Take any unusual wear patterns as a clue, and get it checked, but before you do, make sure the inflation pressures are correct, as incorrect inflation can also cause uneven wear.
The different types of wheel alignments are front-end alignment, thrust angle alignment, and four wheel alignment. In a front-end alignment, the front only is checked. This is fine in some cases, but are the front tires properly positioned in front of the rear tires ? With the thrust angle alignment, that is checked so that the wheels are "squared" to each other. This would eliminate the "dog tracking" that you may have seen on a car that appears to be going down the road with rear end a foot over from the front. The best way to align the car is a four wheel alignment. This will not only do what the thrust angle alignment does, but also includes adjusting the settings on the rear of the car as well as the front.
Not all cars are fully adjustable, but some are. The measurements that need to be checked are caster, camber and toe.
Toe refers to the difference in distance between the front and the rear of the tires. If the distance between the tires is less in the front than it is in the rear, it is referred to as toe-in. It would be what could be commonly called "pigeon toed". If the distance is greater between the front than it is in the rear, that would be toe-out.
Camber describes the amount the tire is tilted away from vertical. A tire has negative camber when the top of the tire leans inward toward the center of the vehicle. Positive camber is when the top of the tire is leaned outward from the center of the vehicle. The camber angle should be adjusted so that the tire is vertical under cornering load. Properly set camber will allow the tire to work at its best, but not have the tire putting too much of its force on the inner edge while moving in a straight line. Tire wear and handling become a compromise. Less negative camber typically will reduce the cornering ability, but give very even wear. Next time you see a photo of an Indy Car, see if you can notice how much camber there is. That is certainly an example of wear not being anywhere near as important as grip.
Caster is the most difficult of the three measurements to describe. If you think back to your bicycle and remember how the tire tilted slightly when turned, that was caster causing the tilt. If you drew an imaginary line through the upper and lower ball joints and compared the angle of difference to a line drawn perpindicular to the ground, the resulting difference is the caster angle. Caster settings allow the manufacturer to balance low speed steering effort and high speed stability. Increasing the amount of positive caster will increase low speed steering effort, but improve high speed stability. Caster also tends to cause an increase in the amount of negative camber as the steering angle is increased.
Regular wheel alignments will usually save you as much in tire wear as they cost. It should be considered routine, preventative maintainance.