Linux is an open source version of Unix. There are many variations of Linux. Anyone can take the source code and modify it to fit their specific needs.
Unix is run on 67% of all websites in the world (Microsoft Server runs on 32.9%, OSX Server on the rest)
of all the Unix servers, 67% is Unix, 35.8% Linux. Debian is the most common flavor of Linux, followed by Ubuntu, CentOS, and Red Hat.
On the desktop, very few people run Linux. For development, I'll run Linux in a VM on my Mac.