Each server-side Web control in an ASP.NET Web Forms application has an ID property that identifies the Web control and is name by which the Web control is accessed in the code-behind class. When rendered into HTML, the Web control turns its server-side ID value into a client-side id attribute. Ideally, there would be a one-to-one correspondence between the value of the server-side ID property and the generated client-side id, but in reality things aren't so simple. By default, the rendered client-side id is formed by taking the Web control's ID property and prefixed it with the ID properties of its naming containers. In short, a Web control with an ID of txtName can get rendered into an HTML element with a client-side id like ctl00_MainContent_txtName.
ASP.NET 4.0 affords page developers much greater flexibility in how Web controls render their ID property into a client-side id. This article starts with an explanation as to why and how ASP.NET translates the server-side ID value into the client-side id value and then shows how to take control of this process using ASP.NET 4.0. Read the entire ASP.NET tutorial.