Assisted GPS, or A-GPS, is a technology that uses an assistance server to cut down the time needed to determine a location using GPS. It is useful in urban areas, when the user is located in "urban canyons", under heavy tree cover, or even indoors. It is becoming more common and it's commonly associated with Location Based Services (LBS) over cellular networks.

A-GPS differs from regular GPS by adding another element to the equation, the Assistance Server. In regular GPS networks there are only GPS satellites and GPS receivers. In A-GPS networks, the receiver, being limited in processing power and normally under less than ideal locations for position fixing, communicates with the assistance server that has high processing power and access to a reference network. Since the A-GPS receiver and the Assistance Server share tasks, the process is quicker and more efficient than regular GPS, albeit dependent on cellular coverage.

One application that I have found that makes great use of the A-GPS data is called Navizon. In addition to letting you use cell tower triangulation to make your normal GPS more accurate, if you don't have a GPS in your phone you can make use of the cell towers to get a rough fix on your position. Navizon also makes use of wifi to try and make your location even more accurate.

This morning on my commute into the city I was able to get Navizon working on my Verizon xv6700 that has no GPS. I was then able to feed that data into Google Maps and see my location. Navizon also comes with mapping software, but it doesn't have all of the features that Google has with local, etc.

It is a good app and you should give it a try on your phone.

Portions of the above taken from the Wiki: http://en.wikipedia.org/wiki/Assisted_GPS