All Mountain Bike Parts Descriptions

#17) Cog Sets - The Cassette. The thing with all the gears on it.
  These clusters of gears, are chosen for their intended use. Small cogs, like an 11 tooth, are for higher speeds, and big cogs, like a 34 tooth will be slower but would have more torque for climbing. Cog sets are available in singles, 5 speed, 6-7-8-9 and now ten speed "clusters". There are two different types, Shimano or Campy. Both use a splined center that slides over a Free Hub Body, and both use a large ring nut to hold them onto the hub. The difference is the spline will only fit it's own brand.
Some of the teeth on the cogs have "pick up ramps" cut into the sides of them to help lift the chain into a lower gear. Others have release cuts to make it easier to drop the chain back down into a higher gear.

Bike Parts Diagram