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 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).
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, as shown in this example:
/userscontains the list of all our users/user/42contains a specific user/user/42/tweetscontains the list of all the tweets associated to this particular user
Maybe your API could allow access to a particular tweet related...