Actually, JBoss is a trademark of RedHat and a brand name used on a suite of enterprise class middleware products.
JBoss Application Server (JBoss AS) is a Java EE-compliant application server, with JBoss AS 4.0.x being compliant with the 1.5 J2EE spec and JBoss AS 5.0 will be compliant with the Java EE 5 spec. JBoss AS 4.2 is an interim measure that provides EJB3 support.
To find out more (especially what modules are contained in JBoss AS), read the documentation available at http://labs.jboss.com/jbossas/docs. Unfortunately, these documents only go up through JBoss AS 4.0.5. So far there are no JBoss AS 4.2 documents, even though that release has been out for several months. JBoss AS 5.0 is in beta so one would not expect documents for it yet.
Thank you very much, PeterJ! I hava downloaded the documents about jboss4.0.2.