Tallow is made primarily from beef or mutton suet and is used in many ways. It's in animal feed, used to make soap, for cooking, and as a bird food. It can be used as a raw material for the production of bio-diesel and other oleo chemicals.
Historically, it was used to make tallow candles, which were a cheaper alternative to wax candles. Many historic soap making recipes call for tallow. When soap is made from tallow it has good lathering properties and is fairly inexpensive.