Doorway pages are a type of web page created exclusively to gain popularity with a specific search engine. They are also referred to as bridges, gateways, and entry pages. They are a matter of contention within the search-engine optimization world, with some holding that their use is both immoral and ultimately counter-productive, and others believing they are a sure-fire way to achieve consistently high rankings.
The underlying concept of doorway pages is simple: a webmaster targets specific keywords to be associated with their site, then builds a number of pages actively targeting those keywords. These doorway pages are not meant for human surfers, but rather for the spiders or robots search engines use to index websites. Many doorway pages make use of a meta tag, refresh, to ensure that human visitors to the doorway pages never even see them. In the past few years, however, many search engines have begun penalizing pages for using the refresh tag, and webmasters have consequently turned to other options.
In the past, doorway pages were quite effective, with search engines rewarding the practice by offering top placement to sites that successfully used thousands of doorway pages to boost their rankings. Recently, though, search engines have begun cracking down on the use of doorway pages, seeing them as a threat to their link relevancy. It has therefore become the opinion of many optimizers that a better long-term strategy is to focus on optimizing the actual content of one's site, rather than building doorway pages to 'fake' the content.
Another major factor in search-engines identifying doorway pages is their nearly identical layout. As a result, a number of webmasters have discovered that search engines are more lenient if they build each of their doorway pages by hand and alter them all. This takes a bit longer, of course, but the payoff is generally considered worth the time.
Lastly, attempting to make doorway pages "human readable" seems to be one of the most important factors in avoiding the ire of the major search engines. It seems that when search engines run across doorway pages which are obviously built only for viewing by robots, they are more likely to delete the offending pages from their indexes and even to ban the main page of the site. This means that pages which consist only of strings of targeted words, rather than some sort of human-readable, useful content which happens to contain each of the words, are something to avoid.
Ultimately, doorway pages seem to be a dwindling phenomenon. They were effective and widely-used when search engines were more naive and less likely to notice such things. But as search engines have matured and their policies have become stricter, tactics such as the doorway page seem destined for extinction.