The UpdatePanel is the workhorse of the ASP.NET Ajax library. It is responsible for defining regions of a web page that trigger partial page postbacks (as opposed to full page postbacks). Such partial page postbacks transfer less information between the client and server and have their user interfaces updated seamlessly, thereby leading to a more interactive user experience. (For more information on UpdatePanels, refer to Using the UpdatePanel.) One side-effect of a partial page postback is that the HTML elements within the UpdatePanel are replaced with the markup returned on postback. This behavior is not noticeable and is not an issue unless you have client-side event handlers wired up to the elements within the UpdatePanel. Such client-side event handlers are lost after a partial page postback.
In short, if there are client-side event handlers attached to HTML elements within an UpdatePanel it is imperative that they be rebound after a partial page postback. This article looks at three different ways to accomplish this. To read the entire article, Building Interactive User Interfaces with Microsoft ASP.NET AJAX: Rebinding Client-Side Events After a Partial Page Postback, click here.