When you place an AJAX request, you also pass in a parameter that mentions the name of a callback function.
The code on the server is designed to return back the json response wrapped inside a function code.
By wrapping the response JSON in a function call, the same origin policy is bypassed, allowing cross domain data interchange.
Inplementing this requires a little extra effort, At both the frontend and the backend.
为了便于客户端使用数据，逐渐形成了一种非正式传输协议，人们把它称作JSONP，该协议的一个要点就是允许用户传递一个callback参数给服务端，然后服务端返回数据时会将这个callback参数作为函数名来包裹住JSON数据，这样客户端就可以随意定制自己的函数来自动处理返回数据了
原文：http://www.cnblogs.com/guojunru/p/5459827.html