Facebook Places is new Facebook feature that allows you to see where your friends are and share your location in the real world. You can now see if your friends are currently checked in nearby and connect with them easily.
You can check into nearby Places to tell your friends where you are, tag your friends in the Places you visit, and view comments your friends have made about the Places you visit.
Here is a video for a good introduction –
At this time, the Places application is available to users in the United States with phones that support W3 geolocation.