What is a clocking and doorway page?

I heard somewhere that clocking and doorway pages are blackhat seo technique and i had read more about it but i did get it so please tell me about those.
Cloaking is the technique in which the content presented in search engine spider is different from that of the information presented to the browser.This is one of the blackhat techniques.Doorway pages refers to the technique in which inserting the result of one particular phrase while sending visitor to different page.
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