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.
Changing a GPS units Datum
Garmin Units (Specifically 60csx but applicable to others)
WGS 84 is generally the default projection for Garmin Units. If for some reason you are in a different projection follow these steps to change to WGS 84.
> Press 'Menu' Twice to bring up the menu screen http://wiki.geocaching.com.au/images/8/89/Garmin.Menu.JPG > Go to 'Setup' http://wiki.geocaching.com.au/images/a/aa/Garmin.settings.JPG > Select 'Units' http://wiki.geocaching.com.au/images/1/1c/Garmin.units.JPG > Then select 'WGS 84' from the Map Datum Field