A Web Map Service (WMS) allows to show over the internet geographic contents stored in a spatial database or file system by using a HTTP protocol. Both a web-based application and a desktop GIS application can make a request (in the form of URL) to the map service which answers with georeferenced map images (usually PNG or JPEG). Some WMS manage the transparency and give detailed information of the showed geographic contents.
The prevalent specification for this standard is published by OGC (Open Geospatial Consortium) and referenced in INSPIRE Directive too. gvSIG and QGIS are excellent WMS client, while mapserver and geoserver are good OGC WMS server. The Sardinia Region exposes a WMS for raster images and one for vectors, with a good user guide (at the moment available in Italian only).