A sleeping bag is a protective "bag" for a person to sleep in, analogous to a bed and blanket. Its primary purpose is to provide warmth and insulation. It also protects, to some extent, against wind, precipitation, and exposure to view, but a tent performs those functions better.
An infant sleeping bag is a bag-like garment or covering worn by infants for sleeping. Infant sleeping bags differ from regular sleeping bags in design and purpose, being designed primarily for indoor rather than outdoor use, and usually featuring either arm holes or sleeves.