Difference between a REST API and a web application

Question

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

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

Answer 1

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.