A reflector telescope uses two mirrors instead of two lenses. Isaac Newton developed this telescope to combat chromatic aberration (a rainbow seen around some objects viewed with a refractor telescope). A mirror used to gather light doesn’t suffer from this effect. Light from an object enters the telescope tube and is reflected off a curved mirror at the end of the tube. A second, small, flat mirror in the middle of the tube reflects this image to the eyepiece.
There are potential problems associated with the mirrors. Firstly, some light is always lost in the reflection; good quality telescopes can usually gather 90% of the light coming in. Secondly, the mirror might not be a perfect curve, so the image being reflected will not come to a perfect point.