Doorway pages have been around for a long time and most people trying for search engine optimization are more or less familiar with them. To recapitulate: a classical doorway page is focused on a certain keyword or search phrase, carrying highly optimized text, pertinent meta tags, title, etc. It is submitted to search engines to achieve better rankings. When human visitors hit page, they are required to click a link to get to site proper.
What's more, some search engines are regularly conducting witch hunts to weed out discernible doorway pages, one of most notable examples being AltaVista. The reason for this is ongoing abuse doorway page technology has been subject to over years. With fairly cheap programs around generating hundreds of doorways at one go, you can hardly blame engines for implementing some self-protective countermeasures.
Another major issue concerns restrictions imposed on volume of page submissions. Search engines tend to limit number of pages a domain may submit per day. While such limits are seldom publicized and may vary widely from one engine to another, a general guideline adhered to by most search engine optimization professionals is about 5 pages per day and domain.
For large web sites generating hundreds or thousands of new pages (dynamic or static) per year, it is therefore paramount to work around this limitation in order to get as many pages indexed as possible. The more web pages represent a web site in a search engine index, greater chances of generating pre-qualified traffic.
Even if you're only selling a handful of products, any web marketing expert will tell you that you are well advised to feature each of them on a highly focused mini-site because this will considerably increase your search engine exposure and, hence, your chances of driving visitors your way. The obvious drawback of this strategy lies in fact that while you may make a lot of one-item sales this way, getting customers to buy several products at one pop isn't quite that easy without diluting your site's thematic focus again.
This is where Doorway Domains come in. They differ from tread-of-the-mill doorway pages in that they constitute dedicated web sites in their own right - highly focused, very targeted and immensely positionable. On other hand, they are not identical with mini-sites because human visitors will never get to see them!
Instead, they are search engine spider fodder at its very best - optimized to last byte, they can achieve excellent rankings. They will usually consist of several pages, each of which is optimized in a different manner: meta tags, titles, keyword density, etc. With every page following a slightly different basic optimization method or algorithm, they can target all important search engines and their various references in an incredibly effective buckshot strategy.