Service Reference in AJAX

Question

Hi.....

                 I am doing a work in ajax and define service reference in scriptmanager. What is service reference in ajax and define its role and process?
<asp:ScriptManager ID="ScriptManager1" runat="server">
    <Services>
    <asp:ServiceReference Path = "~/WebService.asmx" />
    </Services>
    </asp:ScriptManager>