I found this post........ Helped me alittle understanding the same question...........
Where does Tomcat fit into J2EE? Thats a fairly open ended question but here goes. Tomcat is a java based web server that supports both servlets and jsp. J2EE is a framework of java based technologies that includes EJB(destributed components), JMS(asynchronous messaging), JCA(back end conectivity), JDBC(database conectivity)as well as many others. JSP/Servlets are one of those technologies.
Tomcat is usefull for devoloping web applications that use JSP/Servlets. If you need additional J2EE functionality you will probably need an Applicaton Server. Sun produce a reference implementation of a J2EE application server, which you have already downloaded and installed. It is called a reference implementation as it is designed simply for testing J2EE technologies and not as an industry standard product.
As far as reading material goes there is no shortage on the web! Tomcat comes with alot of nice examples of both JSP and Servlet applications. There is a full J2EE step by step tutorial, which you can download from Sun. There should be enough in those two to keep you busy for a while!