To track an iPhone using GPS, first enable Find my iPhone locator services in the settings menu. Then, if the phone is lost or stolen, log into iCloud and switch the phone into lost mode. This causes the phone to periodically broadcast its location using its GPS receiver.
Putting a phone in lost mode brings up a map of its current location in iCloud. If the phone is not currently able to connect to the Internet, it broadcasts the next time it comes within range of a Wi-Fi or cellular signal. It periodically updates its position, and the owner can click on the car icon in the top-right corner to generate driving directions to the phone's current location. If the phone is unable to triangulate its position using GPS, it may still provide its approximate location via Wi-Fi location services.
In addition to GPS locator services, Find my iPhone allows the owner to lock a stolen iPhone. This prevents anyone from using or re-registering the device without entering a passcode first. The program also allows the owner to wipe any personal information on the phone remotely if recovery seems unlikely. Users can also send messages to the phone's screen, allowing the owner to provide contact information for the finder of the phone.