It is specifically written for a certain search engine. For instance, many people use doorway pages to get their site into Alta Vista. They create separate pages with specific words and tags in them, specifically designed for Alta Vista. The page that they create may not have any viewable text on it, it may just be an image with an alt tag behind it. If you see a page, with very little text and a big image, it is probably a doorway page.
Article explains step by step how to create a doorway page.
Doorway pages are designed for search engines. They focus on getting a high ranking for one or two keywords in specific search engines. Links from the doorway lead the searcher to the main site. They are called doorways because they act as an entrance to the site from the search engines. A hallway page links to many doorways for a specific engine.