There are other ways of describing APIs. For SOAP APIs, there are WSDL files that show methods of inputs. For REST there is WADL, but even its apparent simplicity has been too complex for some. At the very least, support for WADL has been minimal. It’s unclear whether Google has created generic enough to work for other API providers. However, it’s obviously something that Google saw the need for, at least internally.
The Google API Discovery API is one of two APIs for APIs.
(Full Story: Google’s API for APIs Makes Sense of Its 80+ Services)
