Traditionally the finder, or finder-scope, is an important part of your telescope. It is a small, low magnification wide field telescope attached to your main telescope tube which is set-up so that the crosshairs that you see when you look through the finder's eyepiece precisely line up with the centre of the field of view of the main scope, even at medium-high magnifications. When you sight an object through the finder you then switch your eye to the main telescope and the object should be somewhere in the field of view. Finders feature on all kinds of telescope and are an essential aid to finding objects that are too faint for the eye to see.
Types of Modern Finder Often Supplied...
These finders won't show you anything you can't see with your eye!
This type of finder was designed to help Go-To telescopes sight on the bright stars they need to set-up. Consequently, they didn't need to magnify or show anything that the unaided eye can't see. However, they became the 'cheap' option for beginner's telescopes and would help you locate bright objects only. As you progress you'll want more than that!
Left: Red Dot Finder types in common use.
The magnifying, light gathering finder.
This allows you to find objects that are too dim for the eye to see.
For example, M27 The Dumb-Bell Nebula in Vulpecula. To find this Messier Object without a finder needs a bit of estimation and a fair bit of luck. The 30mm finder would show it as a small, fuzzy blob that can be put on the cross-hairs.