A sleeping bag is a protective "bag" for a person to sleep in, essentially a blanket that can be closed with a zipper or similar means, and functions as a bed in situations where a bed is unavailable (e.g. when camping, hiking, hill walking or climbing). Its primary purpose is to provide warmth and thermal insulation. It also protects, to some extent, against wind chill, precipitation, and exposure to view, but a tent performs those functions better. The bottom surface also provides some cushioning, but a sleeping pad is usually used in addition for that purpose. A bivouac sack (bivy) is a waterproof cover for a sleeping bag that may be used in place of a tent for lightweight travelers or as a backup if inclement weather occurs.
A basic sleeping bag is simply a square blanket, fitted with a zipper on two or three sides, allowing it to be folded in half and secured in this position. The second major type of sleeping bag, sometimes called a mummy bag because of its shape, is different in a number of important ways.