Just to add to what was mentioned earlier:
GPS approaches are considered "non-precision," as opposed to "precision." Precision approaches give you glide slope information - vertical guidance, as well as aligning you the the runway. This vertical guidance brings you to a "DA" or decision altitude - decision because that's the altitude that you decide whether to land (if you have the required visual references in sight) or execute a missed approach (if you cannot see the required references - see FAR 91.175.) Some examples of precision approaches are ILS, Precision Approach Radar, Microwave Landing System, etc.
Because non-precision approaches have no vertical guidance, the have what is called an "MDA" or minimum descent altitude in lieu of a DA. Once inside the FAF (final approach fix), the pilot descends to the MDA, and if the required visual references are not in sight by the missed approach point (determined by timing, DME, or crossing a navaid) then the pilot must execute a missed approach. Examples of non-precision approaches are GPS, VOR, VOR/DME, NDB, TACAN, etc.
And as was mentioned earlier, the fixes that constitute a GPS approach (IAF, IF, FAF, MAP, etc) are determined by a GPS reciever, rather than radio signals from ground based navigation aids.
The "A" (or B, etc) designation in a non-precision approach means that there are no "straight-in" landing minimums, which means that the procedure track is more than 30 degrees in direction from the direction of the runway - therefore, only circling minimums are given. It should be noted that it is legal to land straight in (on the nearest runway) on a letter designated approach if the appropriate visual refereneces are in sight in time to make a normal approach and landing. It should also be noted that circle-to-land procedures are risky at best due to the combination of proximity to obstacles, low altitude, and poor visabilty. But they are still legal.