Richardson's maturity model
Leonard Richardson is famous for having defined four levels, ranked from 0 to 3, that describe the level of "RESTfulness" of a web API. Each level requires additional work and investment in the API but also provides additional benefits.
Level 0 – HTTP
Level 0 is really easy to reach; you just have to make your resource available on a network through the HTTP protocol. You can use any data representation you find best suited for your use case (XML, JSON, and so on).
Level 1 – Resources
Most people think of resources when they hear the term REST. A resource is a unique identifier for an element of our model, a user or a tweet, for instance. With HTTP, a resource is obviously associated with a unified resource identifier URI, ...