WGS84
From GeoWiki
WGS84 stands for World Geodetic System 1984 datum.
Most countries have developed their own reference frames over time. Unfortunately each datum is slightly different. WGS84 is an agreed worldwide datum which allows people to use the same coordinate system anywhere in the world. This makes it very useful for aviation, shipping, and of course, Geocaching!
Technical information can be found at the WGS84 Website.
