Services in this level provide multiple URI-based resources and operations using different HTTP verbs, and, the most important, hypermedia constraints. According to Roy T. Fielding, a truly RESTful API is one that is driven by hypertext (http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hypertext-driven), which is done to follow the hypermedia constraint. A good example is PayPal's RESTful API (https://developer.paypal.com/docs/api/overview/).

The Richardson Maturity Model provides a good way of thinking about the basic ideas of RESTful APIs. Most RESTful APIs fall into level two, mainly because the gain of being truly RESTful is not worth the overhead of implementing the hypermedia constraint. Or people do not really agree with the benefits brought by HATEOAS. Sometimes, people call level three services pure RESTful and level two services pragmatic RESTful.