Basically, the “a” is for “assisted.” As the tutorial explains, often cellular network towers have GPS receivers. Those receivers are constantly pulling down satellite information and computing the data. This data is then passed on to the cellular phone on request. Thus, your cell phone doesn’t have to identify satellites and make GPS computations because it’s already been done. This makes your cell phone’s GPS faster and saves its battery, which GPS is known to drain.
So far so good. But as the tutorial explains, how aGPS is implemented differs with carriers. For example, AT&T only recently enabled aGPS. Since I’m an AT&T wireless customer, that’s cool.