GPS is Global Positioning System which contains 32 operational satellites orbiting the earth at an altitude of 20,180km from six orbital planes. GPS satellites sends precise orbital information, time, satellite health info etc to the earth in the form of standard GPS frames at 50 bits per sec. The overall GPS Message will contain 37500 bits and it takes approximately 12.5 minutes for the GPS receivers to get the info at 50bps. To get a position fix all this 37500bits of information is not needed. But receiver needs the Ephemeris and Time information and can take minimum of 18 seconds in ideal condition to approx 1minute to get this Ephemeris and time details. This information will be used for calculating the receiver position. As said above Standalone GPS can take greater than 18 seconds or more depending upon the signal strenght etc to calculate position fix, this is a long time for the user to wait for his position.
There comes AGPS, ie Assisted GPS which gets this Assistance data (ephemeris,time,almanac etc) from servers across the world which has an up-to-date assistance info already available from GPS. So through Data connection/Wifi etc the receiver mobile will contact the server and get the assistance data of only those visible satellites for that receiver. This will help to get the position fix done in less than 5 seconds.
Advantage of AGPS- Faster Position fix