A sexual aid, the butt of many parodies and jokes, and, truth be told, a rather distasteful thing. The most basic models are little more than vaguely female-shaped hunks of inflatable rubber with an orifice between the legs, and sometimes another in the face for a mouth.
More advanced models might have a better wig, pubic hair, wholly internal or insertable vibrators, and sacs to fill with warm water for a more "life like" feel. The Cadillac of blow-up dolls, the Real Doll(see www.realdoll.com) is wholly latex, with poseable joints, high-quality wigs, make-up, and are completely user-designable. They're also very expensive.