The non-core, hot-deployable services are added to the deploy directory. They can be either XML descriptor files, *-service.xml, or JBoss Service Archive (SAR) files. SARs contain both the XML descriptor and additional resources the service requires (e.g. classes, library JAR files or other archives), all packaged up into a single archive.
Detailed information on all these services can be found in the JBoss Enterprise Application Platform: Server Configuration Guide, which also provides comprehensive information on server internals and the implementation of services such as JTA and the J2EE Connector Architecture (JCA).