The architectural simplicity from Web plus the use of a widely implemented protocol (HTTP), it's a great platform for an ecosystem of services.
REpresentational State Transfer, describe Web as distributed hypermedia application whose linked resources communicate by exchanging representations of resource state. (from Chapter 1 - Rest in Practice Book)
All REST interactions are stateless. Each request contains all information necessary for a connector to understand the request, independent of any requests that may have preceded it. (from Chapter 5 - Rest in Practice Book)
Definition from REST dissertation:
The model application is therefore an engine that moves from one state to the next by examining and choosing from among the alternative state transitions in the current set of representations.
Representations contain URI links to other resources that might be of interest to consumers.