Traditional Doorway Pages are web pages that have been created for the sole purpose of ranking highly in the search results for one or two searchterms, and are not normally an integral part of the website. If search engines didn't exist, neither would doorway pages.
Some search engine optimizers complain that doorway pages clutter up the search results and, therefore, devalue them for surfers. That view is not without a factual basis, but only in the past. It used to be true that a site could dominate the top results with doorway pages, but that was a failing of the search engines. It used to be possible for site to have 5 or more doorway pages in the top 10 results, and sometimes it was much worse. For instance, when localized Altavistas first appeared, some sites' doorway pages would occupy the top 30, 50 and even more results.
Altavista cleaned up their act and good search engines don't display more than two pages from the same site in the search results for a given searchterm. So doorway pages from a single site cannot clutter up the results. They can fill the top rankings, of course, but not all from the same site. Some people claim that this is cluttering up and devaluing the results for surfers by reducing the relevancy of the results, but they really aren't thinking straight. What difference does it make if a site's doorway page or content page occupies a top ranking? None. A relevant page from the site is listed; that's all. It doesn't devalue the results in any way. Let me give a practical example.
A hotels site offers hotel reservations in hundreds of cities across the world. They decide to promote each location in the search engines and pick on the searchterm, "[city] hotels", where [city] is replaced by each city in which they have a hotel; e.g. New York hotels, San Francisco hotels, Florida hotels, etc. If they include cities and states, and do it for several countries, there will be thousands of variations of the same searchterm. So they make a doorway page for each of the thousands of variations.
Let's say that every doorway page is successful, and makes it into the top 10 rankings for its searchterm. So now the first page of search results for "New York hotels" contains the 'New York hotels' doorway page, and the first page of search results for the thousands of phrase variations also contain a new doorway page.
Let's go even further and let's say that another 9 hotels sites do the very same thing, and with equal success. Now we have all the top 10 rankings for each searchterm occupied by doorway pages, one from each site.
Does this clutter up the rankings? Has it devalued them by making them less relevant? Of course not. Each set of search results contains links to very relevant sites. Click any one of the results and you'll go to a site that offers exactly what you are looking for. You're happy.
So where's the problem? Actually, there isn't a problem but some pseudo search engine optimizers invent problems because they are unable to compete with real search engine optimization, and some search engines invent problems because they want to index the 'natural' web and not a web that has been modified because they exist. That's something they can never have, of course, because as long as search engines display their results in the present form (10 at a time), website owners will always try to ensure that their websites are displayed at the top of the relevant results - where they belong.
Caution: I have shown there there is nothing at all wrong with doorway pages either for surfers or for the engines but, because search engines don't want pages that are not created as genuine content pages, sites using doorway pages can be penalized if found. The biggest risk is associated with auto-generated cookie-cutters; i.e. doorway pages that are all identical except for their target searchterms. They are not difficult for search engines to spot automatically, and I advise against using them.
This concludes our series on SEO Spam - hopefully the lessons you've learned will help improve your site's rankings.