JSONP is just JSON wrapped in a callback function.

JSONP is useful for making cross domain requests, which are often otherwise forbidden by browsers for security reasons.

 // an example of JSON
 {"weapon":"nunchucks","headband":"yellow"}

 // an example of JSONP
 myCallback({"weapon":"nunchucks","headband":"yellow"});

Then to access the data stored in your JSONP, you define the callback function:

myCallback = function(data){
  alert(data.weapon);
};