Web Map Service
A *Web Map Service request* is a way of asking a special kind of web server to create a custom map image for you. A *Web Map Service* is the special kind of web server that handles these requests.
The WMS specification is defined by the OpenGeospatial Consortium (OGC). This group is comprised of many different companies and government organizations, and has the goal of creating public standards which will help make geospatial data easy to access and integrate into existing applications.
The WMS specification has been revised many times. The OGC originally released version 1.0, and has since released versions 1.1, 1.1.1 and (most recently) version 1.3.0
In its simplest form, the WMS allows one to craft a special URL, which will be interpreted and eventually returned as an image of the map you have requested.