Web Forms Server Controls Controls You Can Use on Web Forms ASP.NET IE Web Controls
Microsoft Internet Explorer WebControls is a powerful set of ASP.NET server controls that facilitate the authoring of user interface (UI) elements that deliver HTML content based on a browser’s capabilities.
These controls, commonly referred to as IE WebControls, include four popular UI elements for use in ASP.NET applications: MultiPage, TabStrip, Toolbar, and TreeView.
While ASP.NET 2.0 includes several controls that function similarly, the IE WebControls are designed to take advantage of the powerful capabilities and features supported by uplevel browsers ( Internet Explorer 5.5 or later versions), while also degrading gracefully to server-side functionality in all commonly used downlevel browsers ( IE versions earlier than 5.5 and non-IE browsers ).
IE WebControls generate Web pages that are tailored for both uplevel and downlevel browsers, and exposes both a client-side and server-side scripting object model.
The following examples illustrate the type of interface that can be created by each of the IE WebControls.
IE WebControls are custom ASP.NET controls and, in many ways, function similarly to other rich ASP.NET controls. The WebControls go further than most rich controls in taking advantage of the advanced capabilities of Internet Explorer 5.5 and later versions.
Custom controls have their own namespace, which for the WebControls is Microsoft.Web.UI.WebControls namespace.
IE WebControls address the need for a single-source authoring solution that delivers working content in all commonly used browsers while taking advantage of the advanced capabilities offered in uplevel browsers.
WebControls use components on both the client and the server. For each ASP.NET control there is a DHTML behavior that exposes its own object model for scripting in Internet Explorer 5.5 and later versions.
When an ASP.NET Web Form authored with the WebControls is requested from the server, the requesting client browser is detected on the initial request. If the browser detected is uplevel, a DHTML behavior is downloaded automatically. This allows for possible use of client-side behaviors independently of the server-side authoring environment to provide a richer client experience.
This is particularly significant when authoring Web pages in a controlled environment, such as on a corporate intranet, where the browser type can be preselected or predetermined, and thereby take full advantage of the enhanced client features supported by the WebControls.
Uplevel browsers render pages with richer interactivity and superior performance, as they reduce the need for server roundtrips. And because the rich client pages can take advantage of DHTML behaviors, they also provide superior layout and rendering, and enable extensive customization.
To illustrate, try browsing the below examples, both of which use the TabStrip with the MultiPage control, in different browser types. Notice that in uplevel browsers, there is no postback that occurs when switching between the different page views. 
IE WebControls simplify the authoring of sophisticated UI elements, in which elegant and powerful pages can be authored using a simple declarative syntax. Although declarative authoring is often sufficient to build a fully functional UI, various programming techniques can also be used.
Internet Explorer WebControls Prerequisites Authoring Web Forms with the WebControls