Automobile Alignment consists of adjusting the camber, caster, and toe to ensure that your car will handle properly and consistently. Alignment is also changed for racing purposes, and is the largest component of suspension tuning, the others being adjustment of ride height, spring rate, and damping. One typically does a "front-end alignment" only. You could also adjust the rear alignment as part of the process, but most stock cars have only a camber adjustment in the rear, if that.
Alignment is usually fairly straightforward, with the caveat that on some vehicles, some aspects of the alignment cannot be modified via simple methods. For more on modifying camber, caster, and toe, see their nodes.
Alignment is carried out with the aid of an alignment gauge. This simple device is essentially identical to a level used for construction. It has a curved ampule of liquid (typically colored water) with a bubble of air inside of it. The ampule is graduated with markings for degrees off true. It will also have at least one indicator to ensure that you can level the gauge itself. The gauge is attached to the hub of the car's wheel, or to the wheel itself, and alignment progresses. One first adjusts the caster, then the camber, and finally the toe.
Camber affects traction more than the other two characteristics. Especially in vehicles with body roll, the fact that the outside of the tire normally sits higher means that when you roll onto the tire you will gain traction rather than losing it. If camber is uneven the vehicle will pull toward the side with more positive camber and there will be uneven tire wear across the tread, although the tire wear corresponds roughly with an increase in tire pressure. Heavy steering response and a tendency of the wheels to hop over bumps are signs of too much positive caster, and light steering which tends to wander are clues of too much negative caster. Caster also causes the wheels to gain negative camber during body roll. Finally, toe-in keeps you going straight; the two wheels pointing at one another mean that slight changes in steering wheel position do not change the overall direction in which the car is being pushed (or pulled, if it's front wheel drive.) Excessive toe causes significant inner tire wear. The front wheels are always toed in (1/8 inch difference or so in the front for the average auto) but the rear wheels are sometimes toed out in the back in some types of auto racing in order to make the vehicle turn more easily.