UNIX® is a class of operating system (OS) developed at Bell Labs in 1969. Today, it is owned as a trademark by The Open Group, which oversees its development and publishes the Single UNIX® Specification. Other operating systems that are based on this OS, or share many features with it but do not comply with the spec, are generally referred to as UNIX-like.
Generally, UNIX® is an operating system that can be run on a workstation or a network server. Such systems formed the backbone of the early Internet, and they continue to play an important part in keeping the Internet functioning. UNIX® originally set out to be an incredibly portable system, capable of allowing a computer to have multiple processes running at once, and with multiple users logged in at the same time.
The interactions in early systems took place through a text input and used a hierarchical file storage system. Although UNIX® has changed since its early development, many commands remain the same, and it is largely recognizable today as the same system it was 40 years ago. Since 1994, it has been owned by The Open Group, which purchased it from Novell. The standard continues to develop, and it has also had a number of popular offshoots that began with its core ideals.
The most famous of these is the Linux® kernel, which has its beginnings as far back as 1983 when Richard Stallman began the GNU project to try to create a free version of UNIX®. Although the project itself had no success, in 1992, Linus Torvalds produced a free version of the kernel, which he called Linux®, and he released it under the GNU license. As a result, while UNIX® remained relatively closed off, Linux® was completely open source. This spurred a great number of distributions of the core kernel.
Although people tend to think of UNIX® as a single operating system, it is actually a broader class of systems that meet a specification. Anyone who has an operating system that meets that spec can use the name, assuming they pay the proper licensing fees. A number of existing operating systems could use the mark if they so chose, although in many cases this would undermine their own properties.
For example, the Apple OSX system meets the spec, and so is strictly speaking a UNIX® system. Similarly, the Solaris operating system is in this class, as are HP-UX, AIX, Tru64, and IRIX. Operating systems, like Linux® flavors or BSD, that have a great deal in common with UNIX® but are not technically UNIX® systems because of either a failure to meet the spec, to pay the licensing fee, or both, are often referred to simply as *nix systems. This comes from a practice in the OS itself of using the asterisk as a wildcard symbol, which can stand in for any character. Although technically "UNIX-like" systems is the preferred term, it is very rarely seen in place of *nix, *NIX, or ?nix.