What are doorway pages?
Wikipedia’s definition about doorway pages:
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, zebra pages (a humorous arbitrary coinage by Jill Whalen of High Rankings Advisor), jump pages, gateway pages, entry pages and by other names. Doorway pages that redirect visitors without their knowledge use some form of cloaking.
How do search engines deal with them?
Does it help to increase ranking ?
This is a myth that doorway pages help in ranking of a site.
Reasons:
1. You will not get the link benefit as these pages are not at all connected to other pages.
2. These pages are hardly indexed
3. This is a spamming technique which will ruin your complete site if found by search engines.
Moral of the story is : Keep away from spamming and give quality content to the visitors from a long term perspective.