ONJava.com: What Is a Portlet, Part 2: "What Is a Portlet, Part 2
by Sunil Patil
'Portlets are web components--like servlets--specifically designed to be aggregated in the context of a composite page. Usually, many portlets are invoked to in the single request of a portal page. Each portlet produces a fragment of markup that is combined with the markup of other portlets, all within the portal page markup.' (from the Portlet Specification, JSR 168)"