A basic API is really just a web interface (data wise), but you send the data in a RESTful way (xml,json,php). You just need to create a system that will respond to requests and provide the needed data.

If you're still stuck look through some of these examples: Vimeo simple API or Twitter API. Really, a good example of an 'api' is any RSS/XML feed, all they are is data which your RSS reader parses and displays.

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