An application server does contain a web server that provides html and static content for web requests (i.e., runs a web site).
But an application server also provides containers for the following:
* dynamic web content (for jboss as, this content is written in Java, and is typically referred to as a servlet/JSP container.)
* enterprise JavaBeans (usually provides the business logic, and acts as the go-between for the database and the servlets/JSPs (you can interact with the database directly from servlets/JSPs, but for larger scale applications you will want to separate these concerns))
* a message server
* a web services server
and lots more (I think I hit the major ones).
If you want to get started, I recommend you look into servlets/JSPs first. And from there get into EJBs.
A good starting book is JBoss at Work by Marrs and Davis, though it is for an older version of JBoss AS and older versions of EJBs and web services, and thus somewhat dated. But the first several chapters do a good job of introducing how to use JBoss AS and the concepts surrounding application servers. A free online book on srvlets/JSPs is available at http://www.moreservlets.com/. Once again, dated, but many of the basic servlet/JSP concepts still hold.
If you want to get into EJBs or Web services, then go with the latest - EJB3 and JAX-WS. They make building applications much easier.
If you come form C++ and are a good coder, you probably won't like application servers with all their overheads. ;o)
Their usage only makes sense in case you are writing stupid "business code".
Babak Sayyid Hosseini