UNIX is a multitasking operating system that is commonly used in research organizations and universities. Multitasking means that it can operate many processes simultaneously. Each of these processes are allotted their own time-slices. In other words, the computer's processor repeatedly services one process after another, a little at a time. This gives the illusion of many processes running simultaneously.
Computer hardware has to be controlled by software.
The kernel is the lowest level of the operating system that interacts and controls the hardware. The next layer of the operating system that lies above the kernel would be the X server. The X server is a large program that supports the graphical interface that we see. Scroll bars, title bars, minimize and maximize buttons are all the responsibility of the window manager. Without the window manager, you would not be able to move your windows around.