Unix is a family of operating systems often used by high-end workstations and server computers. Unix operating systems use a modular design, with a primary system kernel, a separate user interface shell, and hundreds of small utility programs. Some versions of Unix are proprietary software, while others are free and open source. It is capable of running on many different hardware platforms.
Unix pioneered several features that are now widely adopted by the computer industry. C was created to write utility programs for Unix and is now one of the most widely-used programming languages. Unix is a multi-user operating system — multiple users can log in at the same time and run programs simultaneously. It uses a hierarchal file system that allows an arbitrary number of nested subfolders. Unix servers hosted many early Internet services, and versions of it continue to run on most servers now.
AT&T developed the original version of Unix at Bell Labs in 1969. They licensed it to businesses, universities, and other organizations in the 1970s and 1980s. Since each licensee could make their own fork of Unix, it ceased to be a single operating system and became a family of similar systems. An industry consortium, The Open Group, currently maintains the Unix trademark and the Single UNIX Specification, ensuring that the numerous Unix versions comply with a set of standards.
NOTE: Common variants of Unix include IBM's AIX, Hewlett-Packard's HP-UX, and Oracle's Solaris. Some other operating systems, like FreeBSD and Linux, are considered Unix-like — they are based on Unix and function the same but do not contain any original Unix code.