In short, Foursquare is a location-based social networking engine. Using the app on your phone you can “check in” to the locations you visit. These check-ins allow you to earn points, badges, and other rewards. Foursquare 3 adds functionality to be able to find specials near you, and leader boards to allow you to compete against your friends.
The bad news is that the Foursquare app for Windows Phone 7 (which is killer by the way) hasn’t been updated yet. The good news is that the fine folks at Foursquare are working hard to get that rectified. Stay tuned here for updates.
Source: Foursquare blog