What is JSON? For what is used for?

JSON (JavaScript Object Notation) is a data storage and communication format based on key-value pair of JavaScript object literals. It is a lightweight text-based open standard designed for human-readable data interchange which is derived from the JavaScript programming language for representing simple data structures and associative arrays, called objects.
In JSON

all property names are surrounded by double quotes.
values are restricted to simple data: no function calls, variables, comments, or computations.
JSON is used for communication between javascript and serverside technologies.