JBoss (i mean JBossAS) is an application server. Eclipse and NetBeans are IDEs. Both are two different things.
IDEs like Eclipse and NetBeans allow you to configure application servers like JBossAS from within the IDE to allow developers to deploy their applications on the server easily without having to do much manual work. However without the application server being available, the IDE on its own cannot provide the features the application servers provide.
JBoss AS uses a version of Tomcat (aka JBoss Web) as its servlet container. You can think of an application server as a superset of the functionality of a web server such as Tomcat.