All Mountain Bike Parts Descriptions

#6) Head Sets - The bearings for your steering.
  Head sets are pressed into the top and bottom of the frames headtube, providing effortless effort....um, to steer the bike. They come in 1" (old style) 1-1/8" (standard) and 1-1/2" (which may or may not be around very long) All headsets sizes can only be used on its corresponding frame. They come in loose bearing, needle bearing, or cartridge, and are available threaded for the older stuff or A-head style for the newer rigs.

Bike Parts Diagram