Why doorway pages used?
 

I heard that doorway pages are very useful to get ranking in less time. Is that true? Should I use them?

Doorway pages are web pages that are created for spamdexing. This is for spamming the index of a search engine by inserting results for particular phrases with the purpose of sending visitors to a different page.

Doorway are sites or pages created to rank highly for specific search queries. Doorway pages are used to improve the ranking of web pages in Search results. The Doorway page can be a keyword filled page primarily for the Search Engines.

Doorway pages are gathering of low quality pages which is created to advance a specific keyword & which have no applicable substance or may have copy content.

Doorway pages are web pages that are created for spamdexing. This is for spamming the index of a search engine by inserting results for particular phrases with the purpose of sending visitors to a different page.
Source - Wikipedia

What Are Doorway Pages?
 

Webmasters are sometimes told to submit “bridge” pages or “doorway” pages to search engines to improve their traffic. Doorway pages are created to do well for particular phrases. They are also known as portal pages, jump pages, gateway pages, entry pages, and by other names as well.