To add to all the excellent replies above, not all variable-pitch props have a constant-speed unit. Although most do, and operate as Dave Incognito and others have described, the aircraft I fly doesn't.
I manually control the pitch of my prop using a 3-position switch. The switch is normally in the centre position, but I can push it up to apply power to an electric motor which pushes the blades into a finer pitch, or down to drive the motor the other way and coarsen the blades.
Having said that, though, the majority of variable-pitch props do have a CSU - mine is definitely the exception rather than the rule.
FFF
---------------