In geodesy and GIS, a geographic coordinate system covering the Earth’s surface, such as latitude and longitude, by which phenomena may be located. When loading a GIS, the correct datum for each map source should be used. When working with more than one map source at a time, all data must be converted to a common datum. The most frequently used datum is the Universal Transverse Mercator (UTM).
www.colorado.edu/geography/gcraft/notes/datum/datum_f.html Website providing an exhaustive list of datums.