It's called Girls Around Me. Even the name of it is a bit alarming. The app uses locating systems to find what girls (or guys) are located near you at any given moment based on Foursquare check-ins.
From there, you can link to a nearby person's public Facebook page to learn information including their full name, age, birthday, school, and anything else they've posted on their profile. Fortunately, the app has been removed from the app store... for now.
Great for maybe a social gathering for finding people who share your common interests, but it so very easily goes past that into the creepy territory.
If you want to get to know someone... why not try TALKING instead of creeping on a phone app?