Portals contain many portlets. Think of a portal as a container for portlets and portlets as containers for content. [ May 13, 2007: Message edited by: Joey Chen ]
Think of the page as the portal, and each section "Search technical library," "Spotlight," as a portlet that each serve unique purposes. [ May 16, 2007: Message edited by: Joey Chen ]
A portal server delivers content to users through individual content containers called portlets.
Here's a pretty thorough tutorial that demonstrates the benefits of a portal, and what exactly a portal is. It's WebSphere Portal specific, but the content pretty much applies to all portals.