Portlet API (JSR-168)

In short, a portlet is an application that makes up a part of the GUI of a portal page.

Portals aggregate different sources, provide SSO, and can be customized (configured). Portlets are pluggable components of a portal that run in a portlet container. The container provides a runtime environment, forwards events to portlets,provides persistence for portlet preferences, and allows access to user preferences. The portal invokes portlets through the portlet container to obtain HTML fragments (the GUI of the portlet) that are assembled to create the GUI.