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.
Doorways are sites or pages created to rank highly for specific search queries. They are bad for users because they can lead to multiple similar pages in user search results, where each result ends up taking the user to essentially the same destination. They can also lead users to intermediate pages that are not as useful as the final destination.
Doorway pages are simple HTML pages that are customized to a few particular keywords or phrases, and they are programmed to be visible only by specific search engines and their spiders.
Doorway pages are typically large sets of poor-quality pages where each page is optimized for a specific keyword or phrase. In many cases, doorway pages are written to rank for a particular phrase and then funnel users to a single destination.