Application Integration is done with providers(portlets)
It depends on the API of your application.
- Basic integration could be a dynalic SSO-Link in a simple provider.
- Some apps via Netlet to remote access the application(wold/excel ...)
- ERP/CRM integration via WSDL or XML/JSP proxy
or even EJB, CORBA or JDBC connectors if your application support these APIs.
For more info about providers see:
http://docs.sun.com/db/coll/S1_PortalServer_60
and
http://docs.sun.com/source/816-6362-10/index.html