Definition - What does Web Map Service (WMS) mean?
A Web map service (WMS) is a standard protocol that describes how to serve any georeferenced map images over the Internet, which is usually generated by a map server that uses data from a geographic information system database. The protocol standard was developed by the Open Geospatial Consortium (OGC) and first published in 1999. WMS provides a simple way of requesting geo-registered map images using an HTTP interface.
Techopedia explains Web Map Service (WMS)
The WMS protocol uses the HTTP interface to make a request. The request defines the geographic layers and areas that the requestor wants to be processed. The response from one of the geographic information systems or one of the distributed geospatial databases to this request is already in the form of an image, such as a JPEG or a PNG image, which can be automatically displayed in a Web browser without any special process, very much like the usual HTTP requests made by the browser to Web servers regarding content. An additional feature to this request system is that the user can request that the images be returned as transparent so that different layers from different servers can be combined together to create overlayed maps that display more information.