Question

$('#<%= txtFirstName.ClientID%>').show();

Trying to send ClientId to external Javascript file using server tags
as a parameter

<input type="text" ID="txtFirstName" runat="server" maxlength="50"
                    class="DefaultTextbox" style="width:180px;"
                    value="First Name" 
                    onfocus="ControlOnFocus('First Name',$('#<%= txtFirstName.ClientID%>').show())"
                    onblur="ControlOnBlur('First Name')"/>

function ControlOnFocus(CompareString,ControlId)
{
    alert(ControlId);

}

Answer 1

$() is a short alias for the main jQuery function, also called jQuery(). You pass it a CSS selector, and in CSS, # means “the HTML element with ID…”. That ID is in the ClientID variable in this program, apparently. The show() call then makes the named HTML element appear.