Doorway pages are optimized pages submitted to search engines to facilitate a redirect. See search engine example above. These are uninitiated ad views that are served to surfers. An impression is served by simply viewing a banner ad, link text ad etc. Advertisers pay per view, not per click-through, to a advertisers web site.
Many providers charge extra for click through to the advertiser's web site.
This type of traffic is used for Branding and e-mail harvesting. Provides low conversion rates and is sold on a CPM basis. Uninitiated page views that surfers do not request. You may have notice this type of traffic when clicking out of a site. Advertiser pays for this impression whether a click through occurs or not.