The Global Positioning System (GPS) is a U.S. space-based satellite system that provides reliable positioning, navigation, and timing services to users worldwide.
If your camera has GPS capability, your photos include GPS coordinates that specify where the photos were taken. When you import those photos, iPhoto reads that information and plots the location of those photos on a map in Places. When you export photos or publish them to the web, you can include this location info.
In iPhoto, you can use Places to add location information to a photo that doesn’t have any.