Web Map Service (WMS) is a standard developed by the Open Geospatial Consortium (OGC) implemented by many geospatial servers, among which we can find the free and open source projects GeoServer (http://geoserver.org) and MapServer (http://mapserver.org). More information on WMS can be found at http://en.wikipedia.org/wiki/Web_Map_Service.
As a very basic summary, you can understand a WMS server as a normal HTTP web server that accepts requests with some GIS-related parameters (such as projection, bounding box, and so on), and returns map tiles forming a mosaic that covers the requested bounding box. Here's the finished recipe outcome using a WMS layer that covers the extent of the USA (source code can be found in ch02/ch02-wms-layers/
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