What's the difference between Ajax and Json in JQuery? When do we use Ajax Or Json?

AJAX  is Asynchronous JavaScript and XML ( technology )

JSON JavaScript Object Notation  (syntax for storing and exchanging data,easier-to-use alternative to XML)

when to use :

you can use AJAX to fetch data from JSON 

JavaScript is client side programming language cant communicate with data base directly the only way is throw third party script witch deliver data as json or xml