You can add a div with runat="server" to the page:
<div runat="server" id="myDiv">
</div>
and then set its InnerHtml property from the code-behind:
myDiv.InnerHtml = "your html here";
If you want to modify the DIV's contents on the client side, then you can use javascript code similar to this:
<script type="text/javascript">
Sys.Application.add_load(MyLoad);
function MyLoad(sender) {
$get('<%= div.ClientID %>').innerHTML += " - text added on client";
}
</script>