The Open Group, an industry standards consortium, owns the “Unix” trademark. Only systems fully compliant with and certified according to the Single UNIX Specification are qualified to use the trademark; others may be called "Unix system-like" or "Unix-like" (though the Open Group disapproves of this term). However, the term "Unix" is often used informally to denote any operating system that closely resembles the trademarked system.

During the late 1970s and early 1980s, the influence of Unix in academic circles led to large-scale adoption of Unix (particularly of the BSD variant, originating from the University of California, Berkeley) by commercial startups, the most notable of which are Solaris, HP-UX and AIX. Today, in addition to certified Unix systems such as those already mentioned, Unix-like operating systems such as Linux and BSD descendants (FreeBSD, NetBSD, and OpenBSD) are commonly encountered. The term "traditional Unix" may be used to describe a Unix or an operating system that has the characteristics of either Version 7 Unix or UNIX System V.

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.

Both Unix and the C programming language were developed by AT&T and distributed to government and academic institutions, which led to both being ported to a wider variety of machine families than any other operating system. As a result, Unix became synonymous with "open systems".

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 collectively known as the Unix philosophy.

Under Unix, the "operating system" consists of many of these utilities along with the master control program, the kernel. The kernel provides services to start and stop programs, handles the file system and other common "low level" tasks that most programs share, and, perhaps most importantly, schedules access to hardware to avoid conflicts if two programs try to access the same resource or device simultaneously. To mediate such access, the kernel was given special rights on the system, leading to the division between user-space and kernel-space.

The microkernel concept was introduced in an effort to reverse the trend towards larger kernels and return to a system in which most tasks were completed by smaller utilities. In an era when a "normal" computer consisted of a hard disk for storage and a data terminal for input and output (I/O), the Unix file model worked quite well as most I/O was "linear". However, modern systems include networking and other new devices. As graphical user interfaces developed, the file model proved inadequate to the task of handling asynchronous events such as those generated by a mouse, and in the 1980s non-blocking I/O and the set of inter-process communication mechanisms was augmented (sockets, shared memory, message queues, semaphores), and functionalities such as network protocols were moved out of the kernel.

In the 1960s, Massachusetts Institute of Technology, AT&T Bell Labs, and General Electric developed an experimental operating system called Multics for the GE-645 mainframe. Multics was highly innovative, but had many problems.

Bell Labs, frustrated by the size and complexity of Multics but not the aims, slowly pulled out of the project. Their last researchers to leave Multics, Ken Thompson, Dennis Ritchie, M. D. McIlroy, and J. F. Ossanna, decided to redo the work on a much smaller scale. At the time, Ritchie says "What we wanted to preserve was not just a good environment in which to do programming, but a system around which a fellowship could form. We knew from experience that the essence of communal computing, as supplied by remote-access, time-shared machines, is not just to type programs into a terminal instead of a keypunch, but to encourage close communication."

While Ken Thompson still had access to the Multics environment, he wrote simulations for the new file and paging system on it. He also programmed a game called Space Travel, but the game needed a more efficient and less expensive machine to run on, and eventually a little-used PDP-7 at Bell Labs fit the bill. On this PDP7, a team of Bell Labs researchers led by Thompson and Ritchie, including Rudd Canaday, developed a hierarchical file system, the notions of computer processes and device files, a command-line interpreter, and some small utility programs.