ASP.NET Code Behind Method is Call by jQuery Function and AJAX.
Let's See How
jQuery.ASPX 
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="jQuery.aspx.cs"
Inherits="jQuery" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>Untitled Page</title>
 <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.3/jquery.min.js"
type="text/javascript"></script>
      <script type ="text/javascript">
          $(document).ready(function () {
              $('#<%=btnjQuery.ClientID %>').click(function () {
                  $.ajax({
                      type: "POST",
                      url: "jQuery.aspx/CodeBehindMethod",
                      data: "{}",
                      contentType: "application/json; charset=utf-8",
                     dataType: "json",
                     async: true,
                     cache: false,
                     success: function (msg) {
                         $('#result').text(msg.d); 
                     }
                 })
                 return false;
             });
         });
     </script>
    
    
    
</head>
<body>
    <form id="form1" runat="server">
    <div>
    <asp:Button ID="btnjQuery" runat="server" Text="call jQuery Function" />
    <br /><br />
    <div id="result"></div>
    </div>
    </form>
</body>
</html>well do not forget System.Web.Services NameSpace and
Method Must be Static and it's Variable also Static.
jQUery.aspx.cs 
 
using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Web.Services;
public partial class jQuery: System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
    }
    [WebMethod]
    public 
static string CodeBehindMethod()
       {
           return "Message from code behind Method.";
       }
}