JSON which stands for JavaScript Object Notation can be defined as a lightweight data interchange format. It is also said a fat-free lightweight alternative to xml. It is a text format which is programming language independent and is native data form of JavaScript. It is lighter and faster than xml. The credit to make json popular goes to Douglas Crockford.
Since JSON is the native data form of JavaScript, it can be used on the client side in an Ajax application more easily then XML.