Difference between a REST API and a web application

What is the difference when I am about to write a REST API?

I mean what considerations I should 开发者_开发百科take into account which are not important for web applications? including the server, programming style, tighter security considerations, etc..

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To me, a web application implies a user interface -- HTML delivered from a server. A REST API, on the other hand, uses the HTTP protocol independent of presentation, and could equally support desktop clients, mobile clients, Flash/Flex clients, etc.

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You should be following RESTful principles as described in Roy Fielding document: http://www.ics.uci.edu/~fielding/pubs/dissertation/top.htm