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);