The word UNIX comes from Uniplexed Information and Computer System (UNICS). The UNIX operating system was designed to let multiple users access the computer at the same time and share its resources. While initially designed for medium-sized minicomputers, the operating system was soon moved to larger, more powerful mainframe computers. As personal computers grew in popularity, versions of UNIX found their way into these boxes, and a number of companies produce UNIX-based machines for the scientific and programming communities.

The key features of UNIX are:

• Multitasking capability: UNIX lets a computer do several things at once, such as printing out one file while the user edits another file.
• Multiuser capability: The same design that permits multitasking permits multiple users to use the computer. The computer can take the commands of a number of users -- determined by the design of the computer -- to run programs, access files, and print documents at the same time.
• Portability: UNIX system is portability, permitting it to move from one brand of computer to another with a minimum of code changes. And new versions of UNIX were backward compatible with older versions, making it easier for companies to upgrade in an orderly manner.
• UNIX tools: UNIX comes with hundreds of programs and tools in two classes: Integral utilities that are absolutely necessary for the operation of the computer, such as the command interpreter, andTools that aren't necessary for the operation of UNIX but provide the user with additional capabilities, such as typesetting capabilities and e-mail.
• Library of application software: there are hundreds of UNIX applications that can be purchased from third-party vendors, in addition to the applications that come with UNIX.

The UNIX system is functionally organized at three levels:

• The kernel: schedules tasks and manages storage;
• The shell: connects and interprets users' commands, calls programs from memory, and executes them;
• The tools and applications: offer additional functionality to the operating system

UNIX Operating System - 1

UNIX Operating System - 2

Reference: http://cbbrowne.com/info/unix.html: The UNIX Operating System