Hi Tony,
It uses the HTML5 geolocation API, a high-level API provided by modern browsers to detect a user's location. It does not only rely on a GPS receiver:
W3C Spec wroteCommon sources of location information include Global Positioning System (GPS) and location inferred from network signals such as IP address, RFID, WiFi and Bluetooth MAC addresses, and GSM/CDMA cell IDs, as well as user input. No guarantee is given that the API returns the device's actual location.
For more information on how to use it, read the documentation
here.