Focusers
Focusers come in many shapes, sizes, and forms. Their job is to hold the "Eyepiece" in the correct position for the image formed by the primary mirror to be sharply formed on the retina of your eye when you look into the eyepiece. In order to do this they must be able to change the distance between the image of the objective and the image on your retina according to the focal length of the eyepiece in use. To accomplish this, either the main mirror is moved up and down inside the telescope, or the eyepiece is moved up and down within a focuser mounted on the telescope.  In "Cassegrain" telescopes the eyepiece is usually fixed in position, and the main mirror is moved. In "Newtonian" telescopes the main mirror is always fixed in position and the eyepiece is moved.
The range of focal lengths for eyepieces is from 2.5 mm to 55.0 mm. This means a movement of 55mm or a travel of 2" is desirable.  You can't go out and buy a focuser for a Cassegrain telescope because the focusing mechanism is an integral part of the telescope's design. For "Newtonian" telescopes there are a large variety of focusers on the market.