Web services that wish to make their data, tools, or features available for developers and clients tend to open up their system through application programming interfaces (APIs).

In the Web 2.0 world, APIs usually follow a RESTful architecture model, where the service will open up a series of URI endpoints to which developers make requests in order to get or set data from that web service system. Depending on the context of the data, these endpoints may be publicly available, or protected through authorization mechanisms such as OAuth.

In any case, these APIs communicate through the HTTP layer: the client will issue an HTTP request to the service, and the service will process the request and return standard response codes.