Doorway pages are web pages created solely for the purpose of attracting the attention of search engine crawlers in an attempt to boost a site's ranking in the results pages of search engines. Doorway pages are sometimes used by search engine optimization firms, but in general it is recommended that you avoid using them at all costs. Since doorway pages serve no purpose except to manipulate the search engine rankings, many engines will actually penalize you if they discover that you are using them.
Doorway pages cannot actually be navigated to through the index page of the website they represent. Rather, doorway pages are intended only to be visible when you search for keywords through a search engine. These doorway pages will have no actual value for the user however, and will only act as click through pages to get to the actual site they are meant to represent.
Nowadays, doorway pages have also become quite ineffective. When they find doorway pages through a search engine, rather than click through, most users will simply opt to go back and find another page through their search results.
So why do SEO marketing firms and SEO "experts" use doorway pages? As you would expect, some search engine marketing firms are much better than others. Often times, less quality SEO firms will rely on automated programs to build sites for their clients, and these often include doorway pages as part of the website structure. These are poor forms of SEO tools, and should be avoided. Other times doorway pages will be used as a way to market sites comprised of lots of Javascript or Flash to the search engines. Again, this is a bad practice.
Overhauling your content, building up inbound links or finding alternate ways to externalize Javascript and Flash are in the end much more effective than using doorway pages.