UNIX in a nutshell

An operating system (or “OS”) is a set of programs that controls a computer. It controls both the hardware (things you can touch – keyboards, displays and disk drives) and the sotfware (application program that you run, such as a word processor).
Some computers have a single-user OS, which means only one person can use the computer at a time. Many older OSes (like DOS) can also do only one job at a time. But almost any computer can do a lot more if it has a multiuser, multitasking operating system like UNIX. These powerful OSes let many people use the computer at the same time and let each user run several jobs at once.Unix operating systems are widely used in both servers and workstations. The Unix environment and the client-server program model were essential elements in the development of the Internet and the reshaping of computing as centered in networks rather than in individual computers

As mentioned above, Unix was designed to be portable, multi-tasking and multi-user in a time-sharing configuration. Unix systems are characterized by various concepts: the use of plain text for storing data; a hierarchical file system; treating devices and certain types of inter-process communication (IPC) as files; and the use of a large number of software tools, small programs that can be strung together through a command line interpreter using pipes, as opposed to using a single monolithic program that includes all of the same functionality. These concepts are known as the Unix philosophy.

There are many different versions of UNIX . Until few years ago, there were just two main versions: the line of UNIX releases that started at the AT&T, and the other line from the University of California at Berkeley. Some other major commercial versions include SunOS, Solaris, AIX, HP/UX and ULTRIX. Some of the freely available versions include LINUX and FreeBSD.