Optics that are out of collimation (with optical elements that are not all exactly aligned on the same optical axis) cannot produce good images. Stars will be elongated or lopsided, planetary details will be hazy and low in contrast, and binary stars will be difficult to split cleanly.
Some telescopes, such as reflectors, can easily be knocked slightly out of collimation, even by the simple act of carrying them outdoors. Catadioptric telescopes (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 mechanical and optical aids that will help you keep the optics of your telescope at their peak to give you the best possible image contrast and resolution every night you go out to observe.