The rotation of the foot downward and inward when you land on the ground is called pronation.

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Some runners, however, overpronate. That is, their feet roll too far inward. This is a common problem that can lead to injuries, particularly of the lower leg and knee. Some runners supinate (or underpronate); their feet roll inward only a little after contact. 

Take the "wet test"
It's a quick and easy way of discovering what your footprint tells you about your degree of pronation.

(a) A normal foot means you probably are a normal pronator.
(b) A flat foot means you may have a tendency to overpronate
(c) A high-arched foot means you may be a supinator.

• Overpronators should wear a shoe with a straight shape.
   

• Supinators should wear a shoe with a curved shape.
   

• Normal pronators should wear a shoe with a semi-curved shape.

"Worry about the fit and the comfort"

A running shoe that fits will be snug but not tight. Buying running shoes that are too small is a common problem. Your running shoes may need to be a half to a full size larger than your street shoes.