There are many different definitions of what a doorway page is. The big defining difference between a doorway page and a page optimised for a particular phrase is the poissible actions for the user, on that page.
Typically a doorway page will have one sole option, and that option will be to go to a single page. The other doorway pages on that site will also have only one single action, and that action will also deliver traffic to the same page. In short, you have one single 'buy me' page and that is your website. So you create 50 keyword rich pages all pointing to that single page and nowhere else.
Content pages on the other hand are a different thing entirely. they are designed to draw traffic into your site through multiple internal links within them. Content pages will be interlinked and will be useful content.
Say you are selling weight loss for example. Your content pages could include diet information, exercise info links to WHO pages, links to other fitness sites etc, and also of course on every page, links to buy your product.
The second is an acceptable set of pages that fit into the web. The first set are just spam.
Fisx has given you the traditional black hat use of doorway pages, and that was that you created a keyword rich page for the search engines, and either cloaked it, or used a redirect for users.