Call webservice from javascript

Hi i was having some difficulty figuring out how to call a webservice asynchronously from javascript. I found somewhere that you had to add the following tag to a scriptmanager:
                <asp:ScriptManager ID="ScriptManager1" runat="server">
                    <Services>
                        <asp:ServiceReference Path="webservice.asmx" />
                    </Services>
                </asp:ScriptManager>
 and add the following t ...