1.question：what is ajax？
Ajax is an acronym for Asynchronous JavaScript and XML（异步JavaScript和XML）。
2.Jesse James Garrett lists the following technologies as the typical(rather than required) enablers for Ajax:
■ Standards-based presentation using XHTML and CSS
■ Dynamic display and interaction using the DOM
■ Data interchange and manipulation using XML and XSLT
■ Asynchronous data retrieval using XMLHttpRequest
■ JavaScript binding everything together
3.difference with ajax：
The main difference between normal requests and Ajax requests is that normal
requests cause the whole browser window (or frame) to be refreshed,showing a blank
page while loading is in progress. With Ajax,requests are done in the background by
an Ajax engine (a script that is part of the page),and responses are interpreted by that
engine and typically used to replace part of the page.
简单来说，使用ajax技术利用异步的JavaScript和XML实现页面的动态局部刷新，
4.principle：
normal：
ajax：
由上图我们可以明白ajax的底层实现原理。
