All waypoints are in UTM format. Wikipedia has a good introduction. UTM is a meter based system, and very easy to use. For example, lets look at the waypoint:
12S 537214mE 4201255mN
12S indicates the coordinate is in zone 12, latitude band S. These are given and cover a fairly large geographic area. For instance, most of southern Utah is in zone 12, latitude S. The mE and mN numbers indicate the meters East and meters North in that zone. If, for instance, your GPS read 12S 537200mE 4201255mN, you would know you are 14 meters (about 50 feet) off from that waypoint.
GPX is short for GPS eXchange format. In technical terms, it is an XML Schema that provides a standard way of storing GPX waypoints, routes, and information.
In much more practical terms, it is a format that many software tools, such as National Geographic Topo!, Babelfish, and others support. This allows you to import the GPX file into Topo!, for instance, see the waypoints on the screen, as well as transfer those waypoints to your GPS without having to type them in.
Lots of new content is being added, with plenty more coming in the future. Thanks for visiting! The site continues to grow far beyond my initial hopes and vision.