Web map server
A web map server (WMS) is a web application which provides portrayal of geographic data which is stored on the server. This data can be stored in a variety of data formats but is served in a limited number of image formats.
This provides the useful ability to restrict access to data since only a picture of the data is sent to the user and the high quality vector or raster data remains safe on the server. This is in sharp contrast to a Web Feature Server (WFS) which returns the features of the data as features. On the other hand it is possible to build thin client applications which only have to show georeferenced images instead of rendering the features.
The Open GIS Consortium publishes a specification for web map servers [1].