The Panel control enables you to work with a group of ASP.NET controls: show or hide : Panel « ASP.net Controls « ASP.NET Tutorial

<%@ Page Language="C#" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">

    protected void btnSubmit_Click(object sender, EventArgs e)
    {
        if (rdlOther.Checked)
            pnlOther.Visible = true;
        else
            pnlOther.Visible = false;
    }
</script>
<html xmlns="http://www.w3.org/1999/xhtml" >
<head id="Head1" runat="server">
    <title>Show Panel</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>

    Select your favorite programming language:
    <br /><br />
    <asp:RadioButton
        id="rdlVisualBasic"
        GroupName="language"
        Text="Visual Basic"
        Runat="server" />
    <br /><br />
    <asp:RadioButton
        id="rdlCSharp"
        GroupName="language"
        Text="C#"
        Runat="server" />
    <br /><br />
    <asp:RadioButton
        id="rdlOther"
        GroupName="language"
        Text="Other Language"
        Runat="server" />
    <br />
    <asp:Panel
        id="pnlOther"
        Visible="false"
        Runat="server">

        <asp:Label
            id="lblOther"
            Text="Other Language:"
            AssociatedControlID="txtOther"
            Runat="server" />
        <asp:TextBox
            id="txtOther"
            Runat="server" />

    </asp:Panel>

    <br /><br />

    <asp:Button
        id="btnSubmit"
        Text="Submit"
        Runat="server" OnClick="btnSubmit_Click" />

    </div>
    </form>
</body>
</html>

3.20.Panel
	3.20.1.	Important properties, methods and events of Panel control
	3.20.2.	Use asp:panel to host asp.net controls
	3.20.3.	The ScrollBars property enables you to display scrollbars around a panel
	3.20.4.	The Panel control enables you to work with a group of ASP.NET controls: show or hide
	3.20.5.	Set AssociatedControlID for asp:Panel
	3.20.6.	Hide, show panel