Doorway pages are web pages that are created for spamdexing. This is for spamming the index of a search engine by inserting results for particular phrases with the purpose of sending visitors to a different page. They are also known as bridge pages, portal pages, jump pages, gateway pages, entry pages and by other names. Doorway pages that redirect visitors without their knowledge use some form of cloaking. This usually falls under Black Hat SEO.
Doorway pages are often easy to identify in that they have been designed primarily for search engines, not for human beings. Sometimes a doorway page is copied from another high ranking page, but this is likely to cause the search engine to detect the page as a duplicate and exclude it from the search engine listings.
More sophisticated doorway pages, called Content Rich Doorways, are designed to gain high placement in search results without using redirection. They incorporate at least a minimum amount of design and navigation similar to the rest of the site to provide a more human-friendly and natural appearance. Visitors are offered standard links as calls to action.
Landing pages are regularly misconstrued to equate to Doorway pages within the literature. The former are content rich pages to which traffic is directed to within the context of pay-per-click campaigns and to maximize SEO campaigns.
Another form of doorway pages are using a method called Cloaking. They show a version of that page to the visitor, but different from the one provided to crawlers, using server side scripts. They know whether it's a bot or a visitor based on their IP address and/or user-agent.
Webmasters that use doorway pages would generally prefer that users never actually see these pages and instead be delivered to a "real" page within their sites. To achieve this goal, redirection is sometimes used. This may be as simple as installing a meta refresh tag on the doorway pages. An advanced system might make use of cloaking. In either case, such redirection may make your doorway pages unacceptable to search engines.
A content rich doorway page must be constructed in a Search engine friendly (SEF) manner, otherwise it may be construed as search engine spam possibly resulting in the page being banned from the index for an undisclosed amount of time.
These types of doorways utilize (but are not limited to) the following:
- Title Attributed images for key word support
- Title Attributed links for key word support
See also 
- What are doorway pages?
- Doorway Pages: Web Site Ranking Aids, or Unethical Shortcuts