A blister is skin that has formed a raised bubble and is filled with fluid. Blisters can form anywhere on the skin in response to shoe rubbing or irritation.
Blisters on the foot are most likely to form on the back of the heel and sides of the foot. Often, blisters are accompanied by discomfort or pain as they rub against shoes.
Shoe rubbing is the friction and abrasion that can occur when loose or improperly fitting shoes rub against the skin surfaces of the foot. Any rubbing on selected spots of the foot can eventually lead to blister formation.