What is the difference between cloaking and doorway page.?

"cloaking" is normally a technique to present another site to search engines than the site a normal, human visitor sees.

The "trick" here is to fool a search engine that the site is of high quality or meets certain criteria (keywords, topic etc.)..so that it ranks well...while "in reality" the site might be a thin site whose only purpose it is to get clicks, CPA sign-ups etc.

A "doorway page" could be defined as a low quality site where the owner doesn't really care about the content/helpfulness of the site...but rather only has an interest that a site visitor clicks on ads or clicks through to a vendor page and buys something there. (For example a low quality affiliate site or a site with adsense ads with low quality content with generic $3 articles on it....or an amazon affiliate site where there is no unique content but rather a copy of existing reviews from amazon with the only purpose to sell something from amazon). Or in a worst case scenario, scraped (dupe) content or unreadable, spun content to display some ads.