Portlets are pluggable user interface software components that are managed and displayed in a web or enterprise portal.
Portlets produce fragments of markup (HTML, XHTML, WML) code that are aggregated into a portal. 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 into any portal supporting the standards.
Portlet-based solutions are an example of component-based software engineering.