The OpenAPI Initiative (OAI) was created by a consortium of forward-looking industry experts who recognize the immense value of standardizing on how REST APIs are described. As an open governance structure under the Linux Foundation, the OAI is focused on creating, evolving and promoting a vendor neutral description format.
How do we know how much Restful API is? Some developers call it “Not Restful API”, some call it “Partially Restful API”, for some, it is “Fully Restful API”, and for some “It is not REST API at all or they call it SOAP based web service”. Imagine it as a spectrum from Not Restful to Fully Restful API. To know at which level API stand, Richardson introduces a model called Richardson Maturity Model. As the name itself suggests, it tells about the maturity level of REST API.