This
http://tomcat.apache.org/tomcat-6.0-doc/config/index.html should get you started with Tomcat specific files.
The deployment descriptor (web.xml) is not Tomcat specific.
It is part of the servlet spec and would be the same in any spec compliant container that you use. As Bear mentioned, the servlet spec is the best resource for learning about this file.
There is a link to the servlet spec in my signature.
-Ben