There is often a lot of confusion around what a doorway page is and since it is specifically against Google’s very own guidelines, it is important to understand that you do not make pages that look and feel like doorway pages. Google’s own guideliens define them as:

They then give these three examples:

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

But John Mueller from Google answered it on video and his explanation will probably be more clearer to you. In short, a doorway page is many pages designed to try to rank for many keyword variations and permutations.

At the 2 minute 9 second mark, John Mueller defined doorway pages as:

Here is the video embed:

Hopefully this clarifies things a bit for more of you.

Forum discussion at YouTube.