In JSON you can have numeric, string and Boolean values. You can also have Arrays and objects. You can also use null to indicate no data. This is fairly restrictive but usually more than enough to code up data to be sent serially across a connection or returned by an API etc.
In a very short time JSON has overtaken XML as the standard data markup for the web. In combination with Ajax and REST it has also overtaken the overly complex SOAP Remote Procedure call.