A Web Map Service (WMS) is a standard protocol developed by the Open Geospatial Consortium in 1999 for serving georeferenced map images over the Internet. These images are typically produced by a map server from data provided by a GIS database.

A Web Map Service (WMS) enables the visualisation of geographic data on the World Wide Web. This standard does not provide the actual geospatial data; instead it just provides a georeferenced image (e.g. PNG, JPEG or GIF files) of the data. 

A WMS:

• Enables the visualization of geospatial information, from various sources and servers across the wold wide web.
• Fulfils interoperability requirements by allowing geospatial users to view varying map sources individually, or overlay multiple views to create customizable maps.
• The interoperability that enables the simultaneous view of map overlays is defined by a set of common interfaces specified by the OGC – OpenGIS Implementation Specification.

A WMS consists of three main operations:

GetCapabilities

Returns information in XML format about the service includng image format and layers, what the server contains.

GetMap

The WMS server creates a ma that can be consumed in 2rd party software like Azimap.

GetFeatureInfo

Allows the query of the WMS for feature information (What data is at this click point?)

Azimap implements all three operations seamlessly so you don't have to worry about how they are setup.

Some search engines for WMS services include:

Geoseer

Spatineo Directory