Expanding on Stann0rz's response, envisioning a master page and content view in action. This demonstration was crafted using ASP.NET MVC but can be easily adapted for traditional ASP.NET Webforms.
MASTER PAGE:
<%@ Master Language="C#" Inherits="System.Web.Mvc.ViewMasterPage" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<style type="text/css">
body {
margin: 0;
padding: 0;
}
div#left {
display: inline;
float: left;
height: 100%;
width: 30%;
background: #A00;
}
div#top_right {
display: inline;
float: right;
height: 30%;
width: 70%;
background: #000;
}
div#bottom_right {
display: inline;
float: left;
height: 70%;
width: 70%;
background: #CCC;
}
</style>
</head>
<body>
<div id="left">
<ul>
<li>Navigation Item 1</li>
<li>Navigation Item 2</li>
</ul>
</div>
<div id="top_right">
<span>Tab 1</span>
<span>Tab 2</span>
</div>
<div id="bottom_right">
<asp:ContentPlaceHolder ID="BottomRightContent" runat="server">
</div>
</body>
</html>
CONTENT VIEW:
<%@ Page Title="" Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="System.Web.Mvc.ViewPage<dynamic>" %>
<asp:Content ID="Content1" ContentPlaceHolderID="BottomRightContent" runat="server">
[Insert bottom-right content here]
</asp:Content>