A datum is a coordinate system based on a mathematical model of the shape of the earth. Datums provide the reference for position and elevation coordinates. A ["horizontal datum"] is a reference for position coordinates, and a ["vertical datum"] is a reference for height coordinates.
Many different datums have been used over the years. The reason for having different datums is that the earth is not a uniform sphere or ellipsoid, and different datums fit different parts of the world better than others. In addition, geodetic measurement technology has improved, and better measurements have allowed datums to be refined.
The NAVSTAR [:GPS:Global Positioning System] that was developed by the US Military in the 1980's and 90's uses a datum, WGS 84, that is a compromise for that technology. Because it is based on satellites orbiting the earth, the datum uses the earth's center as the origin of the coordinate system. Because it is used worldwide, its [:Ellipsoid:ellipsoid] shape is based on a statistical best fit to the worldwide ["geoid"].
The ["NGS"] uses a slightly different datum, NAD 83, for geodetic use in the United States. The differences between NAD83 and WGS84 are slight, but may amount to a few meters in certain parts of the U.S. The differences are due to slightly different measurements that were used to calculate the location of the earth's center of mass.