In modern hardware store vernacular a bow saw is a metal-framed saw in the shape of a bow with a coarse wide blade. This type of saw is also known as a swede saw and a buck saw. It is a rough tool used for cross-cutting branches (maybe upto 6" in dia.) and the like down to size.
The traditional meaning of a bow saw is a woodworking tool used for straight or curved cuts. In European vocabulary it is synonymous with frame saw. In English and American vocabulary it denotes a toothed blade suspended between two long narrow handles called "cheeks" that are supported and separated by a narrow stretcher in the center of the handles, making a wide H shape (the cheeks form the uprights of the H, the stretcher the crossbar of the H). The blade is kept in tension with a turnbuckle or a twisted cord that runs parallel to the blade between the two cheeks but on the opposite side of the stretcher. If a cord is used, the cord is twisted with a toggle attached to one loop of the cord, adding tension. The toggle hits the stretcher, which keeps the cord from untwisting. A finer version of the saw uses a narrow blade (1/4" or less) with handles that allow you to hold the saw and rotate the blade. In this context it is also known as a turning saw.