There are lots of difference between Servlet and Portlet. For your simple understanding, when use Servlet [Simple Web apllication] and if we refresh any page the whole page get refreshed, but in case of Portal Application it only refresh fragment of Portal page, because each Portlets added on the Portal page is independent to each other.
For better understanding, Lets consider a yahoo's web site, where there are more than one application are aggregated on one page. Login and weather. Both are independent application.
Hope this help to understand. Any other thoughts are welcome.