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Pronation is simply a normal foot in motion, from foot-strike on the outside of the heel through the inward roll of the foot. Pronation occurs as the foot rolls from the outer edge to the inner edge. Everyone pronates, and the initial pronation is considered an important and healthy response to the intense amount of shock imposed upon the foot and is integral to propelling you forward. If the foot pronates too much or too little and does so frequently, several biomechanical problems may result that will cause a decrease in performance and increase the possibility of injury.

Pronation is split into three types: Over-pronation, under-pronation, and neutral-pronation.  Over-pronators roll their foot excessively inward on the natural foot-strike.  Under-pronators, meanwhile, hardly roll their foot at all, causing the body to absorb most of the shock of landing.  Use the diagrams below to determine your foot type.

The are two very simple ways of determining your foot's natural pronation. 

1. First, have a friend lie down about 15 feet away from you (a video camera on the floor also works).  Then, walk as normally as possible, barefoot, toward your friend, and have them watch your foot to determine its pronation.  Consult the above diagrams in determining the pronation, and the shoe suggestions below to find the right shoe for your foot type.
2. If you've been running for a while, the easiest way to determine your foot type is to look at your running shoes.  Place the shoes side by side on a table, toes pointing away from you. Look at the shoes from eye-level and you should be able to see one of three scenarios... (1) If the shoes have a slight or significant inward tilt, then you exhibit an over-pronation running gait.  (2) If the shoes appear to be neutral and do not tilt inward nor out, then you exhibit a pronation-neutral running gait.  (3) If the shoes have a slight or significant outward tilt, then you exhibit an under-pronation (supination) running gait.

Source: New Balance Web Express

Overpronators should wear shoes with a straight shape. Overpronators should also look for a motion control shoe, which prevent the foot from rolling too far by providing maximum support. Shoe manufacturers usually include a medial post or rollbar to accomplish motion control. The medial post is basically a solid piece of material on the inner part of the shoe that resists the inward roll of pronation.  Stability means a less flexible midsole, so the shoe won't twist around as easily. The two usually go together.

Underpronators should wear a shoe with a curved shape. Orthotics are also a popular method of support for athletes who have flat feet. Underpronators should also look for a cushioned shoe to allow the feet to roll inward and absorb shock.

Neutral Pronators should look for a running shoe with a semi curved shape. Stability shoes also offer a blend of cushioning and support that is beneficial to people with normal arch and pronation.

Source: Wikipedia