To get the best images out of a telescope, its optics must be collimated (aligned at the proper angles to the light path). Some telescopes, such as reflectors, can easily be knocked slightly out of collimation, even by the simple act of carrying them outdoors. This reduces the amount of fine detail that can be seen through the scope. Catadioptric scopes (Schmidt-Cassegrains) are less prone to lose their collimation, but should be checked regularly to maintain peak performance. Refractors rarely need collimation, unless they have been treated roughly.
Recollimating most optics to restore the best images is a relatively simple task, and should be done on a regular basis. The collimating tools below are optical aids that will help you keep the optics of your telescope at their peak.