The Portlet JSR-168 spec implements a interface similar to
Servlets, although it has a concept of Actions and Renderers (check out the spec) instead of doPost/doGet. If you understand servlets and
JSP it's not difficult to understand.
The main benefit to organisations is that portals can provide a common browser based interface (standard look and feel), and also provide SSO to a number of disparate backend systems, which enables the user to remember one password rather than 10 different passwords.
Other benefits are security, portals provide JAAS Authentication and also the ability to assign content to a user's role, therefore allowing access to content required to perform the job function.
In terms of employment, most of the large enterprise players IBM, BEA, SAP etc have all invested heavliy in their portals, and whilst I don't believe a portal is a solution for every customer, for large scale customers I have seen recent adoption to a portal strategy.
regards
Jason.