Rawhide is dried animal skin that hasn't been treated and turned into leather. Lampshades and drum heads are sometimes made from rawhide, and so are those leathery chews you give your dog to gnaw on.
To turn an animal hide into something strong and pliant (like boot laces, upholstery, or shoes), it has to be tanned, processed with chemicals that strengthen the material. Rawhide, on the other hand, is raw, or "in its natural state." This means it's more delicate and generally less useful than finished leather. The fact that rawhide stretches and softens when wet makes it ideal for drum heads. Dogs love to chew on rawhide, though many vets discourage it.