From Astrodocs

Web Map Services (WMS)

What is WMS?

WMS stands for Web Map Service, and it was developed by the Open Geospatial Consortium (OGC) to allow maps to be served to clients over the Internet via the HTTP protocol.

A WMS server that conforms to this standard accepts queries for map layers and returns the map images that were requested in an image format (e.g. JPEG, PNG). The layers that are returned can be specified to be transparent so that multiple layers can be stacked on top of one another.