What is a Portlet?

Portlet is a small application to fulfill the requirement of certain domain. Inter portlet communication is supported under Liferay portal. Portlet support hot deployment feature of Liferay Portal. 

Liferay portal comes with different portlets by default. These portlets are nothing but different small applications running in the instance(portal). Portlets are pluggable software components that are managed in a web portal.  

In general words you can call them as different catalogues as portlets produce fragments of markup code that are aggregated into a portal. So a portal page is displayed as a collection of portlet, these multiple small applications together making a page of the site and these all pages together makes one community or enterprise portal. Hence a portlet (or collection of portlets) resembles a web-based application that is hosted in a portal.