Basically: You go to a spot (eg, restaurant, bar, bookstore) and check in there. It broadcasts your location to your friends. The difference here is that the

Basically: You go to a spot (eg, restaurant, bar, bookstore) and check in there. It broadcasts your location to your friends. The difference here is that the vast, vast, vast majority of this is all done on the iPhone and your friends get a push notification when you check in. For me, Foursquare is a way to invite others - it's great to see friends checking in somewhere, ping them and join up with them (or vice-versa). Good nights out. The problems with it (personally) arise when people check in.. everywhere. I don't care if you're at work or at home - checking in carries with it an implicit invitation. It definitely works best in large urban areas with lots of activities (I use it in Portland, OR).