JBoss is designed to support sophisticated Java applications that require complex processes such as Enterprise Java Beans (EJB) and/or must have the potential to handle a very high load of usage. Also, many developers choose to take advantage of special capabilities of JBoss that are not available in the JSP servers. JBoss has taken market share away from high end Java application servers including BEA WebLogic and IBM WebSphere.
Note - If all you need is JSP and Java servlet support, you should choose Tomcat. JBoss is very powerful, but it is more complicated than Tomcat and also consumes considerably more memory and processing resources.
More information about JBoss can be found at http://www.jboss.org/
