Viewlets are parts of the page in Plone page rendering process. You can create, hide and shuffle them freely.
Viewlets are parts of the page in Plone page rendering process.
Viewlets are view snippets which will render a part of the HTML page. Viewlets provide conflict-free way to contribute new user-interface actions and HTML snippets to Plone pages.
http://docs.plone.org/4/en/develop/plone/views/viewlets.html