As stated on the OAC’s Web site: 

Annotating is a pervasive element of scholarly practice for both the humanist and the scientist. It is a method by which scholars organize existing knowledge and facilitate the creation and sharing of new knowledge. It is used by individual scholars when reading as an aid to memory, to add commentary, and to classify. It can facilitate shared editing, scholarly collaboration, and pedagogy. Over time annotations can have scholarly value in their own right. Yet scholars remain dissatisfied with the options available for annotating digital resources. Scholars wanting to annotate have to learn different annotation clients for different content repositories, have no easy way to integrate annotations made on different systems or created by colleagues using other tools, and are often limited to simplistic and constrained models of annotation. The importance of annotating as a scholarly practice coupled with the real-world limitations of existing practices and tools supporting annotation of digital content has had a retarding effect on the growth of digital scholarship and the level of digital resource use by scholars.

The overarching goals of this project (consisting of multiple phases) are:

• To facilitate the emergence of a Web and Resource-centric interoperable annotation environment that allows leveraging annotations across the boundaries of annotation clients, annotation servers, and content collections. To this end, interoperability specifications will be devised.
• To demonstrate through implementations an interoperable annotation environment enabled by the interoperability specifications in settings characterized by a variety of annotation client/server environments, content collections, and scholarly use cases.
• To seed widespread adoption by deploying robust, production-quality applications conformant with the interoperable annotation environment in ubiquitous and specialized services, tools, and content used by scholars — e.g.: Zotero, AXE, LORE, Co-Annotea, Pliny; JSTOR, AustLit, MONK.

(via: DigitalKoans)