The document describes the Annotation Ontology (AO) which was developed based on Annotea for annotating different types of documents like text, images, audio, and video. AO defines different types of selectors to specify the context within documents being annotated. It also defines annotation types that are subclasses of the main Annotation type like Comment, Erratum, Question, and Explanation. AO supports grouping annotations into sets and combining annotation sets for specific documents. It aims to integrate with Annotea and SIOC standards for annotations.