Basically, JBoss Web is a servlet container, whereas JBoss AS is an application server. JBoss AS uses an embedded version of JBoss Web to provide servlet support - thus Jboss AS contains JBoss Web.
What do you mean by "use both ... together"?
Thanks for the update.
Got some clarifications regarding JBoss Web.
So, what is the difference between a web server and an Application server ?
Web servers serve web pages Application Servers provide applications and support for those applications. In jboss it provides transaction / services/ ejb etc... If you need a servlet container and not the features that jboss provides go with Tomcat until you do need JBoss AS or have Jboss provided feature set requirements. For web server look at Apache httpd. Application servers are powerful enterprise ready solution stacks to things you need if you need them (when you need them) . If you just need to serve servlets Tomcat is a great container!