Basic AJAX Call

...Javascript tag

<script type="text/javascript" src="js/jquery-1.4.1.js"></script>

<script type="text/javascript">
$('#btnClick').click(function(){
var name = "Nepal";
$.ajax({
                type: "post",
                async: false,
                url: "/WebService.asmx/displayName",
                data: JSON.stringify({ name: name }),
                contentType: "application/json; charset=utf-8",
                dataType: "json",
                success: function (data) {
                alert(data.d);
                 }
           });
});
</script>

...WebService.asmx

<%@ WebService Language="C#" Class="WebService" %>

using System;

using System.Web;

using System.Web.Services;

using System.Web.Services.Protocols;

[WebService(Namespace = "http://tempuri.org/")]

[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]

// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 

 [System.Web.Script.Services.ScriptService]

public class WebService: System.Web.Services.WebService {

    [WebMethod]

    public string displayName(string name) {

        return "Hello "+name+"!!";

    }

}