Area 51 is a codename given to an installation located on the southern shore of Groom Lake in the Nevada Test and Training Range operated by the U.S. Air Force. Whilst the existence of the Area 51 installation (also referred to as Dreamland, Groom Lake, Paradise Ranch among others) is well known, what goes on there is a closely-guarded secret, however it's well known that there is a huge underground complex beneath the Area 51 installation. Some even suggest a transcontinental underground railway system.
But ask almost any conspiracy theorist and they'll tell you a number of stories; the remains from the Roswell crash (and other UFO remains) were taken to Area 51, that the government is building their own UFOs based on alien technology, that aliens are being held there (or even running the show), that they're developing vast energy weapons based on Tesla's work, that they're working on time travel or teleportation, that they have machines to control the weather to fake Global Warming, that it's part of the ECHELON network or that it's the operated by the New World Order or othery shadowy organisations such as Majestic 12.