.json is JSON's filename extension.
The GeoJSON format differs from other GIS standards in that it was written and is maintained not by a formal standards organization, but by an Internet working group of developers.
A notable offspring of GeoJSON is TopoJSON, an extension of GeoJSON that encodes geospatial topology and that typically provides smaller file sizes.
JSON-LD (JSON for Linking Data) is another flavor of JSON in the form of a lightweight Linked Data format. JSON-LD was created so it is easy for humans to read and write, based on the already successful JSON format and providing a way to help JSON data interoperate at Web-scale. JSON-LD is an ideal data format for programming environments, REST Web services, and unstructured databases (like CouchDB and MongoDB).