In contrast to an ordinary telescope, which produces visible light images, a radio telescope "sees" radio waves emitted by radio sources located anywhere in the Universe, typically by means of a large parabolic ("dish") antenna, or arrays of them.
The best-known (and largest) radio telescope is in Arecibo, Puerto Rico. A well-known radio telescope being an array of antennae is the Very Large Array (VLA) in Socorro, New Mexico. The largest (100-meter diameter) and most famous radio telescope in Europe is in Effelsberg, Germany. A typical size of the single antenna of a radio telescope is 25 meter. Dozens of radio telescopes with sizes comparable to that are operated in numerous radio observatories all over the world.
The sub-field of astronomy related to observations made through radio telescopes is known as radio astronomy.
Radio telescopes make our understanding of the Universe better because many celestial objects such as pulsars or active galaxies (like quasars) produce large amounts of radio-frequency radiation and so are best "visible" or even only visible in radio domain of electromagnetic spectrum.
Radio telescopes are also occasionally involved in SETI and tracking space probes (see Deep Space Network).