UNIX is a multi-user, multi-tasking OS. UNIX was first developed at AT & T Bell Laboratories as an OS for minicomputers (around 1969), and after that, several OS was derived through various improvements, and there is now many OS named UNIX. UNIX runs on many computers, including large computers, workstations, and PCs. Linux and FreeBSD are often used as UNIX for PC.

The main advantages of UNIX are that many utility programs are convenient for program development and that there are a lot of network-related functions. The user interface was originally a command input from the keyboard, but recently, many have adopted a window system. UNIX is mainly used for research and development and server applications, as well as for personal use.