A portlet is a separate module within a portal site that 
has a specific function, or displays a separate piece of 
content such as news feeds, stock quotes....etc

A portlet is a program which allows a portal to access 
another program or information. There is often more than 
one portlet for a given application or information set, 
linking the portal to a different view of the application 
or piece of data. The HR-XML consortium focuses on the 
payload specifications for HR, while Employease Composite 
Services refer to the APIs and tools needed to manipulate 
the payload

Portlets are pluggable user interface components that are
managed and displayed in a web portal. Portlets produce
fragments of markup code that are aggregated into a portal
page. Typically, following the desktop metaphor, a portal
page is displayed as a collection of non-overlapping portlet
windows, where each portlet window displays a portlet. Hence
a portlet (or collection of portlets) resembles a web-based
application that is hosted in a portal. Some examples of
portlet applications are email, weather reports, discussion
forums, and news.

Portlet standards are intended to enable software developers
to create portlets that can be plugged in to any portal
supporting the standards.