get请求方式 :请求实体直接拼接在URL后面
ajax.open("get","ajax?name=张三&age=12");
ajax.send(null);
post请求方式 :请求实体需要单独发送
ajax.open("post","ajax");
ajax.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
ajax.send("name=张三&pwd=12");