JBoss AS 7 is a Java application server - it supports the Java EE 6 Web Profile and a future version (AS 7.1) will support the full Java EE 6 specification. So it supports all the standard Java enterprisey stuff like transactions, messaging and security and supports Spring-based apps. as well. But it's also super-lightweight and modular - think Tomcat, Jetty or Resin so small enough to embed or deploy in small footprint environments.