A portlet has several phases.. two of them are action and render. Every phase has request and response. When you first time open a portal page in browser it goes to render phase and then renderRequest and renderResponse come into picture. If you submit a form, click on a link.. you can set its action/href to point to a actionURL which will take you to action phase where actionRequest and actionResponse work. Render phase will always come after Action phase.
You should probably read what is portal framework and what are portlets and portlet lifecycle.
