A doorway page exists solely for the purpose of driving traffic to another page. They are usually designed and optimized to target one specific keyphrase. Doorway pages rarely are written for human visitors. They are written for search engines to achieve high rankings and hopefully drive traffic to the main site. Using doorway pages is a violation of the Terms Of Service of most search engines and could be grounds for banning.
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.
These are pages that regular visitors cannot see, but search engine spiders can. They are used to trick the search engines so that the site gets a higher ranking. Not only do they go against the rules, but they also hurt the visitor's experience. This is a black hat seo technique.