UNIX is one of the oldest Operating Systems (OSs) around. Most of the computer vendors have come up with their own preferred flavors of this widely used OS. But copyright limitations such as only AT&T being allowed to use the name of UNIX as part of their product names have hindered this process to some extent.
There are many variations of UNIX out there, including but not limited to Solaris of Sun Microsystems and OpenSolaris, HP-UX of Hewlett-Packard (HP), the MAC OS X and the OpenDarwin which are based on the FreeBSD, the AIX of IBM, the BSC UNIX of the University of California at Berkley, the FreeBSD which is BSD based UNIX and developed by contributors, the OpenBSD which is BSD based UNIX and developed by contributors, and the NetBSD which is BSD based UNIX and developed by contributors.
Then there are the Linux distributions, including but not limited to Ubuntu which is Debian based and sponsored by Canonical, the Red Hat Enterprise Linux which is commercially released by Red Hat, the Fedora Linux which is sponsored by the Red Hat and developed by contributors, the Debian Linux which is developed by contributors, the SUSE Linux by Novell and the OpenSUSE, the Mandriva Linux which is a far-off commercial fort of the 1990 Red Hat, and the Slackware which is the oldest survivor of all the Linux distributors.
This shows that many companies have their UNIX versions. Linux is also just that: a UNIX variation that was originally meant for Intel PCs which were inexpensive. With so many UNIX versions available, there are also many computer professionals trying it out. The latter versions are easier to use due to simpler installation, configuration, and the use of Web browsers and GUIs. With the Grand Unified Bootloader (shortened as the GRUB with impunity) it is possible for the computer to boot with both Linux and Windows. Most Linux distributions come with a Live CD/DVD for this particular purpose.