Let me add a bit to this. GPS Coordinates are actually Latitude/Longitude Coordinates - in use looooong before GPS satellites (or any man-made satellite) ever existed.
Latitude/Longitude (Lat/Lon) is a spherical coordinate system. UTM is spherical surface projection to a 2D flat surface. However, the Earth is not a perfect sphere. There are different models of the earth that each have varying accuracy in different regions of the Earth. And these models have also improved over time. The "Vincenty" formulas are used to go back and forth between the spherical model (Lat/Lon) and the flat model (Northing/Easting/Zone). The "Vincenty" formula are generalized to allow the use of different "ellipsoidial" models of the Earth using a "data set"of which WGS85 is the most common in use today, at least in the USA. The iPhone GPS chip provides location in Lat/Lon. A particular mapping program will locate a position on its maps based on the UTM data set the maps were created with. For example, Google uses WGS84 as did the old Microsoft Teraserver. I would expect that all map services located in the USA use WGS84. However, there are some map services with data that originates from older printed maps that used NAD27.