Area 51 is the internal US Government codename for the US Air Force's classified facility at Groom Lake in Nevada, about 80 miles north of Las Vegas. it has been the site of several secret projects, most notably the various stealth bombers and fighters produced in the 70s and 80s and used by the US in several conflicts. The site is heavily guarded and access to the surrounding area has been more tightly controlled as time has progressed.
The site attracts great interest from military and espionage buffs, but the secrecy surrounding the facility also attracts conspiracy theorists. Most notably there is a constant belief that the site has been involved in the study of extra terrestrial beings and technology. No real, substantial evidence of this has ever been shown, but there have been several faked photographs and videos purporting to show this kind of activity there.
Despite this lack of concrete evidence an entire culture has built up around the base and its many stories. The most famous example of this is probably the Little Ale Inn, a bar/restaurant on one of the main roads near the entrance track to the base.