OpenLayers
Remember this from Chapter 3, Exploring the Web Interface and Demos? Building web-based maps is the inspiration for the book. OpenLayers is an open source JavaScript library to display web-based maps, similar to the mapping client from Google Maps and a growing number of others. OpenLayers is also a project of the Open Source Geospatial Foundation (OSGeo).
LeafLet (http://leafletjs.com) is a promising mapping client with ties to OpenLayers. Be on the lookout for examples using LeafLet in future chapters.
You'll notice several options at the top of the OpenLayers preview after you click the Options icon. Some of these options are specific to GeoServer, and not part of the WMS specification.