Liferay have several ways to modify the functionality of a native Liferay portlet and you can extend the functionality.

Liferay provide most hooked functionality is described by using the liferay-hook.xml file located in /docroot/WEB-INF directory.

Here are the most common methods are shown :

Custom JSPs

Add the following child to in liferay-hook.xml file.

<custom-jsp-dir>/META-INF/custom_jsps</custom-jsp-dir>

Above element defines the location of custom jsp page's.
For example you may want to rewrite view.jsp for the Document Library portlet at:

[custom-jsp-dir]/html/portlet/document_library/view.jsp

Model Listeners

Model Listeners, In which you will have to define a portal.property file located at,

/docroot/WEB-INF/src/portal.property

The following is an example for the above :

<portal-properties>portal.properties</portal-properties>

If you want to listen to changes in User, for example, you would write in the property.

value.object.listener.com.liferay.portal.model.User=com.my.example.UserListener;

As following format :

value.object.listener.[class-to-listen]=[my-listener-class]

Your class should implement com.liferay.portal.model.BaseModelListener.

Here, You can listen to events such as Add, Update, Remove etc.

Extend\Overwrite Service

A similar here, in liferay-hook.xml in the element add

<service>
    <service-type>com.liferay.portal.service.UserService</service-type>
    <service-impl>my.example.service.UserServiceImpl</service-impl>
</service>

Your implementation should extend the correct wrapper class for a particular service.
For the example above shown below:

com.liferay.portal.service.UserServiceWrapper;

You should now be able to overwrite or extend all the public methods for the UserService like updateUser(..).

Customization of Struts Actions

In very similar as extending services, define the elements for

<struts-action>
    <struts-action-path>/message_boards/view</struts-action-path>
    <struts-action-impl>my.example.action.SampleViewAction</struts-action-impl>
</struts-action>

You should need to extend BaseStrutsAction class.

com.liferay.portal.kernel.struts.BaseStrutsAction