J2EE itself contains lots of stuff. it actually contains a whole lots of Java technology. how about all XML related topics, SOAP, well EJBs of course, JSP topics such as custom tags, servlets, web services, and so on. I recommend you to go through J2EE tutorial that will give you all you need to know.

but the most important  fact in J2EE is how you design and develop you I encourage you to have a look at J2EE design patterns too as you go.

start with java.sun.com

BTW, join www.javapassion.com to learn J2EE in online class, It's free.

there are a lot of technologies  involved, but I
just want to mention that j2ee  is not equal to EJB. this is a trap
which most new commers fall into. ejb is a very important  technology
in j2ee but it has also some alternatives like JDO and JavaSpaces.
the importance of the technologies depends on nature of your project.
it is not that much elegant to use a tank to kill a mouse!

In conformation of the previous postings, I have seen JMS being used while rmi
was a much
better alternative, and TCP/IP ( socket programming ) would have been a perfect
alternative.

For anybody interested in learning J2EE I would suggest learn and develop a very
deep
understanding of computer network programming in Java first before starting
J2EE.

Once you have a clear understanding of computer network programming in Java, you
will
find J2EE very easy to learn and understand.

I would suggest start with TCP/IP ( socket programming ), then learn rmi, JDBC,
IDL,
JNDI, JMS, etc.

I found following one of the best to learn from.

http://java.sun.com/docs/books/tutorial/

I would recommend to go through this site
http://www.ideas2work.com

and click once the google ad as a measure of enhancing this site financially.