The "Rack and Pinion" focuser pictured at the left is one of the earliest designs that has survived because it is simple yet effective.  The eypiece is inserted into the "draw tube" where it is tightened in place by 3 "thumb" screws. The focusing knobs are at both ends of a longitudinally grooved shaft, called a pinion. This pinion is engaged with a geared "rack" mounted on the outside of the draw tube so that it fits into a vertically grooved channel in the focuser's "housing".  Whenever the pinion rotates, the draw tube is forced to move up or down within the focuser's housing according to the direction of rotatation of the pinion. A large thumb screw on the barrel of the housing can be used to "lock" the focusing adjustment in the focused position.
Rack and Pinion focusers come in two major sizes and two styles.  Most amateur telescopes until very recently have a draw tube with an inside diameter of 1.25". The current trend is towards focusers with a draw tube that has a 2" inside diameter.  These can accomodate both the newer 2" eyepieces, and with an adapter, the large variety of 1.25 eyepieces currently available. Of course the outside diameter of the draw tube closely matches the inside diameter of the focuser's housing to prevent any wobble.
The two styles are: "regular" and "low profile".  These styles apply to other "types" of focusers as well.