GNU/Linux handles desktop sessions differently than Windows. A low-level program called the X server provides a bulky but stable interface between application programs and the kernel. What one sees on the screen is the product of a collection of programs called a desktop environment (DE). There are over half a dozen desktop environments for GNU/Linux; the two most popular are KDE (Kool Desktop Environment) and Gnome (Gnu Network Object Model Environment). Each DE can be customized at least as much as a Windows 9x desktop, and with some hacking the appearance and behavior of the login session can be customized completely. Users may customize their own sessions; in fact, a single user may use different desktop environments for different login sessions.