Hi,
See https://github.com/droolsjbpm/jbpm/blob/5.3.0.Final/jbpm-persistence-jpa/src/main/resources/META-INF/persistence-JPA2.xml
That's a persistence.xml configured to use JPA 2 and Hibernate 4 (which is what AS 7 uses by default, I believe).
JPA is the specification for how Java should interact with "persistence", which basically means interacting with a database.
JPA 1 is a couple years old, and with JBoss AS 7, the default way to do things has moved to JPA 2. Within the context of an application on an application server, you can just think of it as a type of dependency.