According to Bare Bones 101, the definition of a Portal:
Portals are directories that have been created or taken over by commercial interests and then reconfigured to act as gateways to the web. These portal sites not only link to popular subject categories, they also offer additional services such as email, current news, stock quotes, travel information and maps.
Some of the best examples of portals are Yahoo, AOL and MSN. These websites want you to use their page as your home page. When you open up your browser – everything is there: mail, weather, news, stocks, etc.. The only problem is that most of the content on the front of these pages are “sponsored links”. The pages are busy and have a lot of ads.
With the invention of personalized homepages like iGoogle, MyYahoo, MyAOL, Pageflakes, Netvibes, and Windows Live – I think we will see less and less use of the traditional portal page. If you already have a Gmail account, you should definitely set up your iGoogle page. It’s fun to add new widgets for mail, calendar, RSS, weather, joke of the day, etc. I get to choose what’s on my homepage. I don’t need MSN to tell me what to read or look at anymore.