jquery ajax为啥获取的data.d是object httprequest

jquery ajax为什么获取的data.d是object httprequest?

$.ajax({

                type: "Post",

                url: "demo.aspx/GetStr",

                //方法传参的写法一定要对，str为形参的名字,str2为第二个形参的名字     

                data: "nickname=" + $("#<%=nickname.ClientID %>").val(),

//                beforeSend: function (XMLHttpRequest) {

//                    $("#nickinfo").text("正在查询");

//                    //Pause(this,100000);

                //                },

                async: false,

                contentType: "application/json; charset=utf-8",

                dataType: "html",

                success: function (data) {

                    //返回的数据用data.d获取内容  

                    $("#nickinfo").text(data.d);

                    alert(data.d);

                },

                error: function (err) {

                    alert(err);

                }

            });    

public partial class demo : System.Web.UI.Page