The Open Annotation Collaboration has released the draft "Open Annotation Core Data Model."
Here's an excerpt:
The Open Annotation Core Data Model specifies an interoperable framework for creating associations between related resources, annotations, using a methodology which conforms to the Architecture of the World Wide Web. Open Annotations can easily be shared between platforms, with sufficient richness of expression to satisfy complex requirements while remaining simple enough to also allow for the most common use cases, such as attaching a piece of text to a single web resource.
An Annotation is considered to be a set of connected resources, including a body and target, and conveys that the body is somehow about the target. The full model supports additional functionality, enabling semantic tagging, embedding content, selecting segments of resources, choosing the appropriate representation of a resource and providing styling hints for consuming clients.
See also the draft “Open Annotation Extension Specification.”