A service archive is just a normal jar archive with a 'sar' extenstion. It is used to packge MBeans and also contains a jboss-service.xml to describe the MBeans.
After you have started JBoss you can copy your archives to the deploy folder and JBoss will automatically deploy them. You can also replace deployed archives and the old deployment will be removed and the new archive will be deployed. This is hot deployment.
POJO is Plain Ordinary Java Object, just a plain Java class that only contains business code and does not require any additional code to make is usuable within the framework you are using it within i.e. No need to implement session / entity bean interfaces.
For AOP I think it will be easier if you just follow the tutoral that describes how it works very well.