Portal - The most straightforward explanation is 'gateway' - from the Latin root of course.
If you've ever been to Paris, the city is surrounded by ways into and out of the city called 'Portes'. Porte d'Orleans for instance. This are a way into - and out of the city. More importantly they - theoretically - make it easier to find your way around (not my personal experience of Paris - but never mind). At each 'porte' or portal you have many directions in which to go to explore the city - or in our case the world of the web.
Practically, this often means that a 'Portal' focuses around a single theme or industry sector and will signpost you to many sites on that same subject. However, just to confuse matters, some of the search engines are also described as 'portals'. MSN, for instance, or Excite are described this way - sometimes some even describe Google as a portal (although I don't personally think that makes sense) but if you look at MSN what you seen is more than just search. Practically speaking then, a portal is a route into a group of web sites and the portal points to them through listing them and through hypertext links to the sites. In principle a portal will have many more links outwards - than inwards - although in fact this is not always the case. In short - a portal is a gateway to the web - pointing you on to other sites.