.Net ServiceHost Directives

1) For WCF Dataservice :
<%@ ServiceHost Language="C#" Factory="System.Data.Services.DataServiceHostFactory"
Service="NwdDataService" %>
Here as we can see that  BwDataService is wcf data service class which exposes Northwind database entities.
Factory property indicates DataServiceHostFactory which has logic of taking an entity and exposing its content using OData pattern.

2) For WCF service :
<%@ ServiceHost Language="C#" Debug="true" Service="Service" CodeBehind="~/App_Code/Service.cs" %>
Here we have path for code behind class file which happens to be Service.cs, this file contains definition of Service class.
Service defines name for wcf service class which has implementation of wcf service class.

3) Web Service
<%@ WebService Language="C#" CodeBehind="~/App_Code/WebService.cs" Class="WebService" %>
Here also we have location of Codebehind file and service class name which defines actual service content.