Back in the old days of search engine optimization, software companies such as WebPosition Gold would sell you a program that autogenerated thousands of doorway pages that were of no value to a visitor. Most SEO’s have long since forgotten about doorway pages, but apparently some black hat search marketers still use them and Google has publicly put these folks on notice.
The revised definiton of doorway pages in the Google Webmaster Help Center.
Old doorway page definition
Doorway pages are pages specifically made for search engines. Doorway pages contain many links – often several hundred – that are of little to no use to the visitor, and do not contain valuable content. HTML sitemaps are a valuable resource for your visitors, but ensure that these pages of links are easy for your visitors to navigate. If you have a number of links to include, consider organizing them into categories or into multiple pages. But in doing so, ensure that they are intended for visitors to navigate the sections of your site, and not simply for search engines.
New doorway page definition
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.
Whether deployed across many domains or established within one domain, doorway pages tend to frustrate users, and are in violation of our webmaster guidelines.
If you string the doorway page words together you get: each page is optimized for a specific keyword or phrase…then funnel users to a single destination. This is the definition of a landing page.
Google obviously hates low quality pages designed to usurp search engine results pages, but focused landing pages are key to high level conversions.