Hmm
A portal is more a site that aggregates information - for example the Yahoo homepage is customisable (a personalised portal) to have, for example, stocks, news ticker, headlines, horoscopes, etc... The aggregation may come from their own sources, or from external sources (e.g. Reuters news).
In terms of the early history of a "portal", they originally started as a directory - I seem to recall altavista working this way ages ago. You'd navigate down the directory tree to find "relevant" web links.
Pretty much any website that offers a directory of "other links" could be defined as a Portal :-)
How it differs from an intra/extranet. An intranet is a web space in a private domain. Most companies will have this in some form - for example, where I work has a huge intranet, consisting of many portal pages (i.e. links to other internal resources that are organised in a categorised fashion - e.g. finance, HR, divions, departments, ...) and a large global file repository (glorified FTP server). My view of an intranet is purely the infrastructure that provides the internally addressible webspace. I do not view an intranet as a portal, although some content in the intranet may be a portal. The intranet is merely a transport mechanism.
The extranet is the outside world - i.e. those web pages addressible outside of the intranet domain. This site (LycosIQ) is an example. Again, I don't view the extranet as a portal (except in the vaguest of senses), but instead just a carrier.
As to your supplemental questions, it all depends on the portal owner. Meta tags in the HTML code have been so thoroughly abused by the porn industry (and everyone else trying to increase their returns on searches)that they are quite untrustworthy nowadays. Google works by working out the associations between words and sites (which leads to the interesting phenomenon of Google-bombing), other sites may purely try and index the content. You can submit a site to Google, or they will eventually trip over it in their automated crawling of the web (especially if you become popular)
