The cowbell is a percussion instrument which is made out of metal and shaped not entirely unlike a cow's bell, minus the rattling thing inside. It is struck with either a wooden stick or a rubber mallet and makes satisfying loud "pock" sound.

Cowbells come in various sizes and are used mostly in Latin and rock music. The most common rhythm seen played by a cowbell is steady quarter notes:
pock pock pock pock pock pock pock pock
This can get horrendously boring to play, but it sets up a very solid groove for other members of the rhythm section to follow.

Concert band music has seen quite its share of cowbell parts too; these parts are usually more complex but less visible than those in Latin and rock music. I've even seen veritable cowbell solos in some pieces:
This is from Ode to Lord Buckley. It is so fast as to require two mallets to play it.

The cowbell is also fun to play, though highly nonstandard and stupid looking, in a marching band.