The website is based off social-networking and allows users to "check-in" into venues (Locations, Buildings etc) and basically play a game where users can collect badges, points, view stats and submit tips about the venues. It's mainly for mobile use only but anyone with a PC and Internet access can "check-in" to a location as long as it's legit.
Foursquare is a very entertaining location based social game. You virtually "check in" at venues in your city, share tips what to do and see what your friends are doing. You can earn all kind of "badges" by participating actively. Recommended!
You can optionally connect to your facebook and twitter accounts in a secure way, but it's up to you.