Cog: 

 One of the rear sprockets that attaches to the hub on the rear wheel.

Cassette:

A group of stacked sprockets on the rear wheel of a bicycle with a rear derailleur.

Chainring:

A large cog carrying the chain on a bicycle, which is attached to the crank.

Crank:

One of  a set of two on a bicycle that converts the reciprocating motion of the rider's legs into rotational motion used to drive the chain or belt, which in turn drives the rear wheel.

Spindle:

An axle around which a pedal rotates; threaded at one end to screw into crank arms.

Derailleur:

An assembly of levers, usually cable actuated, that moves the chain between sprockets on a cassette or chainring assembly.

Gearing:

The aspect of a bicycle drivetrain that determines the relation between the cadence, the rate at which the rider pedals, and the rate at which the drive wheel turns.