The differences between pronation and supination are that pronation is the term for the inward roll of the foot in motion and supination refers to its opposite, the outward roll of the moving foot. Supination is also known as underpronation.
Pronation refers to the inward rolling movement of the foot while walking or running, specifically the moment when the outer edge of the heel comes into contact with the ground and the foot rolls inward to distribute the force from the impact. Some pronation occurs naturally as the foot moves, but if it happens to an excessive degree, it can flatten out the arch of the foot, leading to problems with muscles, ligaments and tendons.
Supination is the term for the outward roll of the foot in motion, when the heel lifts up and the front half of the foot and toes push off from the ground to propel the body forward. During supination, the force of the impact from the ground is distributed a lot more irregularly throughout the foot than during pronation, placing excess stress on the ankles and toes especially. Although supination is also a natural movement of the foot, excessive amounts can lead to Achilles tendinitis, iliotibial band syndrome or total ligament rupture.
Runners need to be aware of these types of motion to determine whether they have problems with excessive pronation or supination. If so, there are exercises as well as special footwear and orthopedic inserts available that can help.