The web service is based on Representational State Transfer, or REST http://en.wikipedia.org/wiki/Representational_state_transfer. This simply means that each resource can be referenced by its own URL, and the actions performed on that resource correspond to HTTP methods. For instance, to fetch data you would issue a GET request for the desired resource with any options specified as query parameters.
There are two distinct environments available for the API. The production environment is the one your production applications should use, but a sandbox environment is also provided for testing your integrations. The sandbox is an environment completely isolated from production, but some of its data is based on the data from the production environment.