<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    <style type="text/css">
        .tooltip
        {
            position: absolute;
            top: 0;
            left: 0;
            z-index: 3;
            display: none;
            background-color: #3AC0F2;
            color: White;
            padding: 5px;
            font-family: Arial;
        }
        td
        {
            cursor: pointer;
        }
    </style>
    <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
    <script type="text/javascript" src="http://cdn.jsdelivr.net/jquery.simpletip/1.3.1/jquery.simpletip-1.3.1.min.js.txt"></script>
    <script type="text/javascript">
        $(function () {
            $('[id*=GridView1] td').each(function () {
                $(this).simpletip({
                    content: GetTip($(this))
                });
            });
        });
        function GetTip(cell) {
            var row = cell.parent();
            return "Name: " + $("td:nth-child(1)", row).html() + " Age: " + $("td:nth-child(2)", row).html();
        }
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">
        <Columns>
            <asp:BoundField DataField="Name" HeaderText="Name" />
            <asp:BoundField DataField="Age" HeaderText="Age" />
        </Columns>
    </asp:GridView>
    </form>
</body>
</html>
 protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            DataTable dt = new DataTable();
            dt.Columns.AddRange(new DataColumn[2] { new DataColumn("Name"), new DataColumn("Age") });
            dt.Rows.Add("John", 25);
            dt.Rows.Add("Rick", 29);
            dt.Rows.Add("Andrew", 11);
            dt.Rows.Add("Peter", 14);
            GridView1.DataSource = dt;
            GridView1.DataBind();
        }
}
 Screenshot
