UNIX is an old (by technology standards) operating system, developed by AT&T's Bell Labs in 1969. Due to it being an open source system, there are many variations of it in use. Linux is a higher level system originally written in 1991 by Linus Torvalds and was built on the UNIX system.
By todays standards, UNIX is not a user friendly system but it is stable and reliable. It's most supportive fans are academics and users of the UNIX based Apache Server because the cost is minimal compared to systems produced by commercial enterprise. For someone who wants to deal with computers at the nitty-gritty level, UNIX is a great way to have that experience.