From what I (think I) know:
- Linux systems can be considered clones of UNIX.
- Most UNIX are commercial, most Linux distros are free and open source.
- Technically, UNIX usually refers to the entire OS, but Linux is really just the kernel.
- The file systems used in Linux are usually ext3 or ext4 in general. In UNIX, it's usually ufs or zfs (go try Solaris), jfs or gpfs, hfs or vxfs, etc.
- The popular Linux distros (Ubuntu, Fedora, openSUSE, etc) are usually more user-friendly than UNIX systems (Mac OS X is an exception).

More info: http://www.ibm.com/developerworks/aix/libr...-difflinux.html