A finderscope is a small, low power, wide-field telescope that attaches to the side or top of a larger telescope by means of a machined or die-cast finderscope bracket that bolts to the larger telescope tube. The finder bracket generally has two narrow rings, each with three nylon or nylon-tipped metal screws that hold the finderscope in place. The nylon-tipped screws also collimate the finderscope (line it up with the main telescope optics so that what you see in the finder crosshairs will also appear in the center of the telescope eyepiece). Some brackets (from Takahashi, for example) have a single long ring with three ball-headed screws at the front to hold the finder and three nylon-tipped screws at the rear to collimate it. Others (from Celestron, for example) have a single long ring with a rubber gasket at the front to hold the finder and three nylon screws at the rear to collimate it.
Some finder brackets bolt to the telescope tube semi-permanently. Others have quick release dovetail mechanisms that allow the finder to be removed quickly for transport and replaced in the virtually same collimation when the scope is set up again.
Some finders are sold with brackets as part of their price. Other finders are sold by themselves, with the brackets as an option.