GeoServer is a GIS server. It is open source and 
it allows users to view and edit geographic data. 
GeoServer is a fully functional WFS-T and WMS 
server that follows the OGC specifications.
GeoServer is a tool you can display maps on web pages, 
where the user can zoom and pan around. And it is 
used in conjunction with clients such as OpenLayers, 
Google Earth, UDig, GVSig and others.

WWW: http://docs.codehaus.org/display/GEOS/Home