A radio telescope is a type of telescope that "sees" things in radio wave frequencies rather than visible light. Whereas a "normal" telescope works by collecting light using a lens or mirror, a radio telescope collects radio waves using an antenna.
There are many different type of telescopes that operate in different wavelengths, including infra-rad, ultra-violet, ex-rays and gamma rays. Radio telescopes tend to operate in the frequencies around 10 cm to 10 meters.
Unfortunately, the use of radio waves by humans is making radio astronomy more and more difficult. Radio telescopes need to be in a "quiet" area away from artificial transmissions. Even in space, Earthly radio noise is a problem. In future, it is likely that serious radio astronomy will need to be done in places shielded from earth altogether, e.g. on the far side of the Moon.