"Portal" is a web site designed to be used as the user's "home page" in web browser settings.
Basically, it's supposed to be the page where the user finds the cool stuff to surf to that day, and get some news. And, of course, probably find some games and other services too. (Much to antispammer's horror, many of generic portals offer free webmail. See also throw-away account.)
There are many "generic" portals that are supposed to be customizable for wider audience, such as My Yahoo!, MSN, My Netscape, and so on, and regional generic portals (such as Kolumbus, Sonera Plaza and Saunalahti.fi for Finland).
Then, there are "special interest portals" that offer information and news from specific area; Examples from IT industry might include Slashdot and Internet.com.
The problem, of course, with portals is that there are so many to choose from; Personally, I have yet to find a portal that would put Slashdot and Kuro5hin headlines and headlines from Euronews, Kaleva and MTV3 news to same page (but AvantGo happily delivers all I can ask for to my Palm)...
Here's a generic advice for webmasters, though: Don't make your page a "portal". It either happens or it doesn't, but never do that intentionally. And an advice for coders: An easy-to-use "portal kit" would be nice, so that every yahoo could be Yahoo!... =)