Google Latitude is a service you can subscribe to using a Google Account that allows you to share your location, in real-time, with others. It utilizes your cell phone or computer's internet connection to determine where you're located and will display it on a Google Map for your friends to see.
Location sharing starts only when both you and a friend agree. You can invite friends via email or easily add them from your Gmail contacts. You can share, set, hide your location - or turn off/sign out of Google Latitude - from a privacy menu. You can also hide your location or share only a city-level location with certain friends. Additionally, you can create a status message and upload photos. It also syncs directly with Google Talk. You can check your friends' status messages to see what your friends are up to and can quickly contact your friends with an SMS, IM, or phone call. You can also get directions to lead you to your friends.