Web Feature Service
The Web Mapping Service (WMS) returns a streamed image to a GetMap request. The purpose of the Web Feature Service (WFS) is to return an XML document with information. The information could be the bounding box (geographic extent) of a feature, or some feature attributes (in GIS terms, typically called an "identify"), or even "return the list of features that are within 1 mile of this point". Data can also be edited on the server using the WFS.
There is a good WFS introduction at the uDIG web site.
--Raj Singh, OGC (December 15, 2006)
Here is a link to a very simple client that uses the GeoTools and OpenGis libraries to connect to the MassGIS WFS. They can be found at www.geotools.org
--Matt Calder (June 1, 2007)