Notebooks are saved as GitHub-flavored markdown documents and stored as gists to your GitHub account.
Notebooks work with APIs described with RAML (RESTful API Modeling Language). The API Notebook can create a client for an API when you specify the URL for the API's RAML spec. With that client you can send authenticated requests to the API and receive live data in return. The API Notebook currently supports OAuth1 and OAuth2 as well as unsigned requests. For more details, please read this guide to APIs and the Notebook.
You can create a RAML file with any text editor and host it anywhere. One convenient way is to use the API Designer on the Anypoint API Portal. For more information, see http://raml.org/projects.html.
You can pull in any web-accessible RAML spec. Simply type "API.createClient(<client name>, <URL to RAML spec>)" in any code cell. To pull in a RAML spec from the Anypoint API Portal, use the "Insert API Client" button above every cell:
Learn more about RAML at http://raml.org.
If you have questions about the API Notebook or RAML, please post them on http://forums.raml.org.