“PageMethods” is the keyword which is used for calling your code behind methods inside javascript function. To implement PageMethods functionality we need to consider three points.

• EnablePageMethods property to true in ScriptManager on your page.
• Calling method should be Static.
• use [System.Web.Services.WebMethod]  on your Static method.

Example :

<script type="text/javascript">
  function Test() 
  {
     PageMethods.TestYourMethod(onSuccess, onError);          
  } 
  function onSuccess(result) 
  {
    alert("Result " + result); 
  }
  function onError(error) 
  { 
     alert("error " + error); 
  }
</script>

 [System.Web.Services.WebMethod]
    public static string TestYourMethod()
    {
        try
        {
            return "Hello";           
        }
        catch (Exception ex)
        {
           return ex.message.tostring();
        }
    }

<asp:ScriptManager EnablePageMethods="true" ID="ScriptManager1" runat="server"></asp:ScriptManager>