Underpronation
Supination (or underpronation) is the insufficient inward roll of the foot after landing. This places extra stress on the foot and can result in iliotibial band syndrome of the knee, Achilles tendinitis, plantar fasciitis. Runners with high arches and tight Achilles tendons tend to be supinators. Shoes will wear on the entire outside edge, and the side of the shoe becomes overstretched. If you place shoes on a flat surface, they tilt outward. Runners with flat feet normally overpronate.

Overpronation
Excessive inward roll of the foot after landing, such that the foot continues to roll when it should be pushing off. This twists the foot, shin and knee and can cause pain in all those areas. If you are an overpronator, you'll find excessive wear on the inner side of your running shoes, and they'll tilt inward if you place them on a flat surface. Knock knees or flat feet contribute to overpronation. High-arched runners typically underpronate. Best suited to guidance / support shoe.

Normal Pronation
The outside part of the heel makes initial contact with the ground. The foot "rolls" inward about fifteen percent, comes in complete contact with the ground, and can support your body weight without any problem. The rolling in of the foot optimally distributes the forces of impact. This movement is called "pronation," and it's critical to proper shock absorption. At the end of the gait cycle, you push off evenly from the front of the foot. If you have a normal arch, you're likely a normal pronator. Best suited to a neutral shoe.