You should check http://www.seologic.com/faq/doorway-pages its explained very well. Hope its helpful.
Doorway pages - 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 Technique.
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.