1) The basic idea behind Seam was a unification of the EJB/JPA and JSF object models. But it has gone much beyond that.
2) Seam is a framework, and you need to download the framework to use it. Just using JPA+EJB+JSF in your app does not mean you are using Seam. And by the way, Seam apps can be deployed to app servers other than JBoss AS.
The book Seam in Action is an excellent introduction into Seam. Looks like chapter 1, which provides an overview, is a free download.