People ask us all the time - what are portals? How can we make out website a "portal" for people? There are a number of definitions of portals, so lets start with the easy ones. The most commonly known portals are those that are large generic collections of information. For instance Google is a "Search Portal". You launch Google, search for your results, then move to the specific site that has the information you require. Such a portal is known as an "aggregator" in the sense that it aggregates data omnivorously then organizes it in a way that is easy to search. Most aggregator sites make their money selling targetted advertising that is either content specific or in some way manipulates the search results in favor of those that advertise with them.
Another type of portal is one that is specific to a particular topic or industry. Again - they will attempt to aggregate data from other sites, or take data from other sites and add value to it. A local example is www.webhomeusa.com. This local site aggregates data from homesteadnet.com's Multiple Listing Service feed, maps from Google and Microsoft Live, and additional data added by the site's author to provide a portal to people who are looking for homes (currently in the Rochester NY area, but eventually throughout the country and Canada). By organizing the data in an easy to find way, and by adding value to the data they provide the service of unbiased results. These industry specific aggregators again promote themselves by driving traffic, delivering free results, in trade for advertising dollars.
The third type of portal is a "user inspired" portal, one that is created to bring together a community of users with a common interest. For instance the game "World of Warcraft" has millions of users around the world. A portal has sprung up http://www.worldofwconline.com - mostly sponsored by users of the game - that allows them to interact together outside the more controlled space of the game and it's official websites. This portal started as a community only portal, then spread to become advertising driven as well. Generally these portals are structured by the fans themselves and are mostly "labors of love" rather than monetary sources.
The final type of portal is one that goes "out" from your site to something that is related but not specific to your website's content. OS-Cubed for instance provides a "portal" from our site to our customer support portal. You can see the login for the portal right within our site, and login and use it....