Collection+json is a hypermedia format used to manage collections and items in a REST api. The format details how data and operations on them can be represented.
Questions about this tag should be about getting help in representing resources with this format.
The complete specification is at this address: http://amundsen.com/media-types/collection/
Here is the main git repo with the work-in-progress draft of collection+json, cj clients, and proposed extensions to the format: https://github.com/collection-json
A good number of examples and a more in-depth description of the format can be found in Mike Amundsens' book Restful Web Apis http://restfulwebapis.com/
See also https://stackoverflow.com/tags/hypermedia/info for a general definition of hypermedia.