There are some differences between Assisted GPS (AGPS) and Differential GPS (DGPS) (you can see in below pictures). Generally, GPS device (GPS receiver) takes some time (5 to 10 minutes) to pick up the satellite signal (especially if you are inside the building etc.) for generating information and this time is called “Time To First Fix” (TTFF). But can we wait for such a long time to receive our location from our phone, no right? So, for reducing TTFF, AGPS is used in our mobile which redirects current location through the mobile phone network, wifi etc. and can reduce the TTFF timing from 5 to10 minutes to a fraction of seconds. Which means our phone use AGPS (which is partial GPS).
But in the case of DGPS, a fixed known receiver is used which calculate the drift (errors) and improve the accuracy in the real time.
A DGPS system can improve the accuracy of GPS from 10 meters to less than one meter.
PS: This article was initially posted on my Facebook wall (22/05/2017).