A datum refers to the reference point from which all maps are drawn. GPS is based universally on a grid for the entire earth that a GPS can interpret anywhere in the world called WGS-84. Unfortunately, many maps were published before GPS so they utilize different datums-reference points. In North America, the datum is called the North American Datum 1927 or NAD27. Be sure your GPS can interpret the map datums for the areas you will be in.