This is right from Google and hope it helps.
Doorways are sites or pages created to rank for specific, similar search queries. They lead users to intermediate pages that are not as useful as the final destination. Examples of doorways include:
Having multiple websites with slight variations to the URL and home page to maximize their reach for any specific query
Having multiple domain names or pages targeted at specific regions or cities that funnel users to one page
Pages generated to funnel visitors into the actual usable or relevant portion of your site(s)
Substantially similar pages that are closer to search results than a clearly defined, browseable hierarchy