Unix is a proprietry system and not free, SCO and other companies still sell
The major difference is that linux (a unix clone) is free under the terms of the GNU license. There are more than 300 linux distributions all freely available to download (you can buy a boxed set but the money usually includes a printed manual and support from the vendor).
Some history in one paragraph:
The legendary Richard Stallman founded the Free Software Foundation / GNU project and wanted a kernel for his new operating system. Linus Torvalds had produced a unix kernel which was just right for Stallmans OS, and hence GNU/Linux was started in 1991. And in case youre wondering GNU stands for GNU's Not Unix.