Buckram is one of those things that most people know nothing about, despite the fact that they have almost certainly handled it many times in their lives. It's a heavy cloth that's used to make book covers, and at one point was used in clothing. Usually it's made of cotton, though linen buckram also exists. In order to make it more durable, buckram is coated and calendared, which is a process that gives the cloth a smooth finish by filling the space between the fibres with some other substance. Normally this other substance is pyroxylin1, but it may also be starch or clay.
Buckram is sometimes specified in sizes, from Number One (quite coarse) to Number Four (very fine). Often buckram for books consists of a sheet of the coarse stuff on the inside with a finer weave glued to the outside. Book buckram is also rated as Group D, E, or F, with Group F buckram being the heaviest.
Some sources claim the name buckram comes from the town of Bokhara in Uzbekistan, where it was first made. Others trace the term to an old French word for goat.