Documentation : Protocols
The Easy API has been engineered to provide access via XML POST, JSON, and GET. Each service that The Easy API offers has it's own documentation which provides sample calls that can be used to access the service.
Depending on the endpoint which is being used (xml.theeasyapi.com, get.theeasyapi.com, json.theeasyapi.com) is what will determine what will be sent back. If using the endpoint xml then valid version 1.0 XML will be sent back. GET produces XML but does not require a POST to obtain the data, and JSON will be ... well ... JSON.