JSON

JavaScript Object Notation (JSON) is - despite the name - not a JavaScript-specific standard. It's a standard for encoding complex data structures into a string. It's useful for passing structured information back and forth between two programs. JSON is taking the place of XML in many cases. It's quicker and easier to read and write with a briefer syntax and less rigid structure. Wikipedia.

CORE uses JSON almost entirely in the context of AJAX calls. Encoding data as JSON before passing it to the client callback allows the server to send more information in each response without adding a burden on the client manually parse the response and rebuild its structure.