A complete guide to modular holds
Modular climbing holds can be thought of as little pieces of a cliff (the part you hold on to) that can be fastened to a wall surface to form the essense of a climbing route. The modular part means you can change these key pieces of cliff around to create many different climbing routes.

I've come up with a quick classification scheme (the M.A.S.S .system) to organize this little discussion of modular holds.
Material - composition (plastic, rock, ceramic, cbc), texture, and other physical properties of the surface of the hold.
Attachment method - bolt-on or screw-on, how the hold gets on the wall.
Shape - the different "types" of hold shapes, and intent (for realsitic or training) use.
Sex - do the holds stick into the wall (inset) or out of the wall (everything else).