TextBoxWatermark is an ASP.NET AJAX extender that can be attached to an ASP.NET TextBox control to get watermark behavior. When a watermarked TextBox is empty, it displays a message to a user with a custom CSS style. Once the user has typed text into the TextBox, the watermarked appearance disappears. The watermark is intended to provide more information to a user about the TextBox without cluttering up the rest of the page.