What is a Servlet Filter? (see Servlet Spec 2.3)
Allen Fogleson A filter is basically a component that is invoked whenever a resource is invoked for which the filter is mapped. The resource can be something like a servlet, or a URL pattern. A filter normally works on the request, response, or header attributes, and does not itself send a response to the client.
[That's great, but a little brief -- anyone want to elaborate with more details, code samples, etc.? -Alex]