The doorway page, the satellite page, the channeling page, the information page, the landing page, the entry page, the bridge page..., whatever you call them they've long been considered the bete noir of seo or search engine optimisation. Everyone's got their own definition but all of them embrace roughly the same principle, a heavily optimised keyword or keyword phrase-riddled page built solely to attract the search engines that then links back to a main web site.
Some of these will be made up of pure and absolute nonsense bar the top paragraph, that will be proper English with a link back to the main site using the keyword or key phrase as anchor text in the backlink or IBL (Inbound Back Link). The same keyword or phrase will similarly appear in the page's title tag, the meta description tag and the header or H1 tag, while the rest of the page will be utter gobbledegook, machine-generated Babel spam with the keyword or key phrase sprinkled randomly through the content just to a density which will appeal to the search engines.
Others will be made up entirely of this machine-generated keyword or phrase riddled spam and will automatically redirect through use of javascript or a meta-refresh to a completely different site. SEO techniques like this, when (not if) discovered will inevitably suffer some kind of search engine penalty if not necessarily an outright ban.
Traditionally it will characteristically also be a page that stands alone outside the main body of a site, existing on it's own as an individual entity. It will have no incoming (IBL) links from the site; any that it does have will probably be coming from series of similar pages, all created with the same aim in mind, to rank highly in all or a particular engine for a particular specific search term.
If you do a search for doorway page in Google amazingly there are still adverts for companies who openly announce their dubious expertise in this frowned upon SEO practice despite the very highly-publicised decision by Google to ban BMW for, among other things, their blatant use of this forbidden SEO technique.
Perhaps a better approach would be to note that now, though, you can legitimately use this basic optimisation technique by making each page into an article and submitting it to an article distribution site. No-one seems to have realised yet, or if they have they haven't drawn any particular attention to it, that while using doorway pages is still very much a Black Hat SEO technique, one that can and has been known to get sites banned, you can exercise good White Hat SEO by creating articles that perform exactly the same SEO function as a doorway page and no-one will complain or even adversely comment. It seems that, in the eyes of the engines, if you hang the label "article" on a certain kind of doorway page it then becomes perfectly acceptable to all.
For instance, is this article linked to from my site? No it is not. Is this article heavily optimised with keywords related to SEO and key phrases related to search engine optimisation? Indeed it is. It fulfils, then, at least some of the criteria exponents of so-called "Ethical SEO" would use to label it as search engine spam, but manifestly, in commenting on the phenomenon it is itself a part of, it isn't spam at all, more (I believe myself) an example of social commentary on the search engine optimisation or SEO industry.
So then, is this article itself a doorway page? Like so much in search engine optimisation, that really depends on your own individual point of view. My advice? Look not at what I say I do, look instead at what results from what I do - if I have a high-ranking stand-alone page for popular SEO keywords with a link back to my main site from it, one that can not be banned for being a doorway page because it is in fact an article that's about doorway pages, what will the consequences actually be?
Strange, Grasshopper, strange are the ways of the SEO!

About The Author

"Big" Bill Kruse occasionally takes time out from philosophical debate to offer SEO Services from his site at http://www.kruse.co.uk/.
kruse@cityscape.co.uk