What are the differences between JSON and JSONP?

Format wise, file type wise and practical use wise?

JSONP is JSON with padding. That is, you put a string at the beginning and a pair of parentheses around it. For example:

//JSON
{"name":"stackoverflow","id":5}
//JSONP
func({"name":"stackoverflow","id":5});

The result is that you can load the JSON as a script file. If you previously set up a function called func, then that function will be called with one argument, which is the JSON data, when the script file is done loading. This is usually used to allow for cross-site AJAX with JSON data. If you know that example.com is serving JSON files that look like the JSONP example given above, then you can use code like this to retrieve it, even if you are not on the example.com domain:

function func(json){
  alert(json.name);
}
var elm = document.createElement("script");
elm.setAttribute("type", "text/javascript");
elm.src = "http://example.com/jsonp";
document.body.appendChild(elm);

Basically, you're not allowed to request JSON data from another domain via AJAX due to same-origin policy. AJAX allows you to fetch data after a page has already loaded, and then execute some code/call a function once it returns. We can't use AJAX but we are allowed to inject <script> tags into our own page and those are allowed to reference scripts hosted at other domains.

Usually you would use this to include libraries from a CDN such as jQuery. However, we can abuse this and use it to fetch data instead! JSON is already valid JavaScript (for the most part), but we can't just return JSON in our script file, because we have no way of knowing when the script/data has finished loading and we have no way of accessing it unless it's assigned to a variable or passed to a function. So what we do instead is tell the web service to call a function on our behalf when it's ready.

For example, we might request some data from a stock exchange API, and along with our usual API parameters, we give it a callback, like ?callback=callThisWhenReady. The web service then wraps the data with our function and returns it like this: callThisWhenReady({...data...}). Now as soon as the script loads, your browser will try to execute it (as normal), which in turns calls our arbitrary function and feeds us the data we wanted.

It works much like a normal AJAX request except instead of calling an anonymous function, we have to use named functions.

jQuery actually supports this seamlessly for you by creating a uniquely named function for you and passing that off, which will then in turn run the code you wanted.