That question is better for Redhat to answer, but you can read about it at http://www.jboss.org/jbossas/faq or just google on "JBoss EAP vs Community".
If you want to be "free" use the community version (recently renamed to Wildfly)
http://wildfly.org
http://wildfly.org/download/ (Not all versions are available here. You may have to build it yourself using the code from https://github.com/wildfly/wildfly)