1. Web Annotations (W3C)
Annotations are typically used to convey information about a resource or associations between resources; they can be described as bridges between different resources that provide a relation among them. Simple examples include a comment or tag on a single web page or image, or a blog post about a news article.
The Web Annotation Data Model specification describes a structured model and format to enable annotations to be shared and reused across different hardware and software platforms without losing meaning and will be extensible when there is a need to add more functionality. Common use cases can be modeled in a manner that is simple and convenient, while at the same time enabling more complex requirements, including linking arbitrary content to a particular data point or to segments of timed multimedia resources.
Web Annotation Data Model does not concern itself with HOW the data will be transferred, as to allow freedom in the used technologies.
This small image from W3C's web annotation page displays the relationship between body of information, annotation and the target.
The specification provides both a conceptual model that accommodates these use cases, and the vocabulary of terms that represents it. A specific JSON format called "RDF" is recommended for ease of creation and consumption of the annotations.