http://footpaininfo.com/biomechanics.html
During heel strike of the stance phase the foot begins to pronate. Pronation of the foot is the term that describes the rolling motion of the foot inwards and flattening of the inner (medial) arch of the foot. Pronation allows the foot to adapt to uneven terrain and absorb the impact of the foot striking the ground. It is during this phase that the foot begins to act like a shock absorber.

During midstance the entire foot is in contact with the ground and the weight of the body is directly over the foot. It is during this phase that the foot is maximally pronated. The foot acts as a shock absorber during the early part of this phase. As the body weight shifts forward the foot begins to return to a neutral position in preparation for heel lift.

Heel lift occurs at the end of the stance phase. Supination of the foot is the term used to describe the rolling motion of the foot outwards and the rising of the inner (medial) arch of the foot. During heel lift, the foot supinates to act as a rigid lever. The plantar fascia is a strong connective tissue that runs along the bottom of the foot connecting the heel to the base of the toes. The bones, muscles and the plantar fascia act together to form this rigid lever.

the wole article describes in detail what a'normal' gait looks like and then addresses the problems that may occurm due to injury etc.
I hope this helps