A blister is an area of raised skin with a watery liquid inside. Blisters form on the feet from rubbing and pressure, but they form a lot more quickly than calluses. You can get blisters on your feet the same day you wear uncomfortable or poor-fitting shoes.
Areas on your body that form blisters and continue to be rubbed every day (like your feet because of the same pair of uncomfortable shoes you always wear to school) may go on to form calluses.
Whether you walk all day long, hike, run, or shoot hoops, blisters are almost as much a part of the action as sweat is--but they don't have to be. You can take measures to avoid them. And if you develop a blister, you have several options for lessening the pain and lowering your risk of infection. To prevent blisters, you need to minimize friction. For the feet, this begins with shoe selection. Shoes should fit comfortably, with about a thumb's width between your longest toe and the end of the shoe. Too narrow shoes can cause blisters on the big toe and little toe. A shallow toe box can lead to blisters on the tops of the toes, while loose shoes can create blisters on the tips of the toes.
If the blister isn't too painful, do everything possible to keep it intact. Unbroken skin over a blister provides a natural barrier to bacteria and decreases the risk of infection. Cover a small blister with an adhesive bandage, and cover a large one with a porous, plastic-coated gauze pad that absorbs moisture and allows the wound to breathe.
Don't puncture a blister unless it's painful or prevents you from walking or using one of your hands. If you have diabetes or poor circulation, consult your doctor before doing anything with a blister. Call your doctor if you see signs of infection around a blister — pus, redness, increasing pain or warm skin.
Source: Mayo Clinic