Quickbooks XML is used communicate with most versions of Quickbooks, including Online Edition
Quickbooks XML is a soap data format that is used to make SDK calls to most versions of Quickbooks. It is preferred over the legacy IIF format because it runs through the API, and is thus constrained to behave more like the QB frontend. QBXML is also bi-directional, while IIF can only insert data.
If you do not have a local Microsoft COM program running on your machine to connect to your instance of QB directly, you can use qbwc to make your QBXML calls via a remote web server.
The complete QBXML reference can be found in the Intuit OSR