Doorway pages - If you want to have a good listing in the search engines, some webmasters will load the home page with lots of keywords. And it's nonsense or useless for the visitor to see. Therefore the webmaster will redirect the visitors to another page that is much easier to read (or have the user click a link to get to that page). This is basically cloaking. And as we now know, cloaking is a no-no.
I will not teach you the Code for automatic Redirecting one page to another because we are studying here how to avoid getting your site banned to google. :D