An operating system tells the computer how to understand what jobs it has to do, how to do these jobs, and how to tell people the results. Millions of computers may be using the same operating system, while each computer can have its own application programs to do what its user needs. Using the same operating systems makes it easy to learn how to use computers for new things. A user who needs to use a computer for something different, can learn how to use a new application program. Some operating systems can have simple command lines or a fully user-friendly GUI.