I only know about portals from a lecture but as far as I know a web portal is a container for widgets. These widgets, referred to as portlets, do stuff like show stuff like your inbox, calculator, search bar, calendar, etc. They were useful because of all the functionality the portlets provided in the portal, meaning that you didn't have to load a bunch of different programs or web pages to access these things individually.
The only 2 places I recall portals were the university computer systems and Google's iGoogle which is being discontinued this year.
iGoogle: http://www.google.com/ig