A terrestrial planet is a planet which is mostly composed of silicate rockss. The term is derived from the Greek word for Earth, "Terra", so an alternate definition would be that these are planets which are, in some notable fashion, "Earth-like". Terrestrial planets are substantially different from gas giants, which may not have solid surfaces and are composed mostly of some combination of hydrogen, helium, and water -- existing in various physical statess. Terrestrial planets all have roughly the same structure: a central metallic core, mostly iron, with a surrounding silicate mantle. Luna is similar, but lacks an iron core. Terrestrial planets have canyons, craters, mountains, and volcanoes.
Earth's solar system has four terrestrial planets: Mercury, Venus, Earth and Mars. At one time there were probably many more terrestrials, but most have been ejected from the solar system or otherwise destroyed. Only one terrestrial planet, Earth, is known to have an active hydrosphere.
NASA is considering a proposed project called the Terrestrial Planet Finder, which will be capable of detecting terrestrial planets in other solar systems. All currently-known extrasolar planets are extremely large, and are most likely to be gas giants.
See also: Jovian planet