The Extension environment (called Ext environment) provides the capability to customize the Liferay portal completely. As it is an environment which extends the Liferay portal development environment, it has the name Extension; its short form is Ext. By Ext, you would modify internal portlets (also called out-of-the-box portlets). Moreover, we would override the JSP files of the portal and out-of-the-box portlets.
Starting from Version 6, the Ext environment is available as a plugin called Ext plugin. Custom code will override the Liferay portal source code in the Ext plugin. In the deployment process, custom code is merged with the Liferay portal source code (that is, it will override files of the portal core). Ext plugins provide the most powerful methods of extending the Liferay portal core, designed to be used only in special scenarios in which all other plugin types cannot meet the needs.