Finderscopes are small, wide-field secondary telescopes mounted on the main telescope.
Many entry-level telescopes come with 5 x 24 finders—these give 5x magnification and have apertures of 24 mm. Their small lenses are fine for centering a telescope on a bright planet. If you want to locate fainter targets, however, consider upgrading to at least a 6 x 30 finder. For owners of 6 to 8 inch (150 to 200 mm) telescopes, an 8 x 50 finder is a better choice.
An accessory some people find wonderfully useful, either instead of or as well as a finderscope, is one of the new reflex finders, such as the Telrad. Looking through a reflex finder, you see a naked-eye view of the sky with a red dot or bull's-eye floating among the stars.