A blister is a localized collection of fluid causing a bump (a raised area) of the skin and separating it into an upper and lower layer. The fluid collected between the two layers of skin can be blood, lymph, or serum (the watery, clear portion of the blood).
Most blisters are on the feet or hands. Blisters can be a result of burns, sunburns, friction (e.g. from shoes), pressure, or hard impacts. Spanking with certain implements, such as paddles with holes drilled through, can cause blisters.
Blisters from paddling develop when the paddle stroke compresses the skin, forcing fluid out of the tissue and into those areas where the holes are drilled through the paddle, resulting in circular blisters. Paddling which damages capillaries can also cause blisters to form, or contribute to their formation.
A blister is different from a welt, which is also a raised area of the skin. In a welt, the skin is not separated in two layers, and the fluid remains in the tissue, rather than forming a pool as in a blister.
A blister, and the area around it, is often reddened from whatever caused the blister.
Blisters are painful to the touch (especially to friction) and can pop, resulting in an open wound that may well need treatment such as disinfection, bandaging, and/or a sticking plaster. Avoid peeling any skin off a popped blister. If a blister is deliberately punctured to drain the fluid (which can relief the pressure, but also cause infections), a sterile needle should be used.
 See also
- How to Care for a Blister on eHow