JSONP is a simple way to overcome browser restrictions when sending JSON responses from different domains from the client.
But the practical implementation of the approach involves subtle differences that are often not explained clearly.
Here is a simple tutorial that shows JSON and JSONP side by side.
It assumes that you are familiar with basic JQuery functions and have basic experience of server-side applications.
Specifically the tutorial uses a simple Sinatra server, written in Ruby
See the page public/index.html for the Tutorial itself.
A live version of the tutorial is at http://json-jsonp-tutorial.craic.com