In biochemistry, starch is a polymeric carbohydrate (a polysaccharide), in which the monomers are glucose units joined to one another usually head-to-tail. The overall structure is not, however, simply a linear polysaccharide chain, since occasionally, two glucose units are joined to one, forming a branch point.
Animal starch is the common name of glycogen. It is not the same as ordinary starch.
Aside from the smooth, crisp edges it gave to clothing, it served a practical purpose as well. Dirt and sweat from a man's neck and wrists would stick to the starch rather than fibers of the clothing, and would easily wash away along with the starch. Then, after each laundering, the starch would be reapplied.