Doorway pages are Web pages designed and built specifically to draw search engine visitors to your website. They are standalone pages designed only to act as doorways to your site. Doorway pages are a very bad idea for several reasons, though many SEO firms use them routinely.
As a rule of thumb, if you can't reach the page by following the site navigation, then it is a doorway page. You are not supposed to "visit" the page. Instead, you are just supposed to find it in the search results and then click through to get to the site in question. In essence, a doorway page is no more than a one-page click-through advertisement for a website. However, when you are searching, you don't want to visit one-page click-through advertisements for a website. You want to visit websites. Think of doorway pages as giant banner ads, only worse. You're searching for "widgets" and instead of getting a widget site, you get a page that says, "click here for widgets." You just did that! Now they're asking you to do it again. Are you going to click?