Difference between Unix and Linux?

Can anyone tell the fundamental difference between UNIX and Linux? I have recently installed Red Hat Enterprise Linux 5 (RHEL5) on VMWARE and found it interesting but I there was one nagging question - How are UNIX and Linux different architecturally? Also, there is a terminal in every linux distro which accepts the UNIX commands. Does it mean that UNIX is an integral part of linux? Anyway, here are some screenshots of RHEL desktop running on VMWARE 6.

Login Screen


Desktop


Terminal

as to my knowledge UNIX is a licensed one unlike linux

more info @ http://www.diffen.com/difference/Linux_vs_Unix

Unix and Linux doesn't differ much in the functionality that they offer. They both have some features that are common and some others that differ. Both of them are used as servers to serve client requests. I haven't found any comprehensive article that stated the differences, but there are some differences that I have noted and thereby learned and would like to share over here,

1) UNIX is an Open Group specification while Linux is an Open Source kernel designed for an Open Source UNIX clone: Gnu/Linux.

2) The source code is not freely available for any of the commercial versions of UNIX. Quite the opposite exists for Linux.

3) Table 1. Filesystems that come standard with UNIX versions
AIX jfs, gpfs
HP-UX hfs, vxfs
Solaris ufs, zfs
Irix xfs

Linux, however, supports almost all of the filesystems that are currently available on any operating system.

4) The Linux versions tend to be based on the GNU version of these tools, whereas the current UNIX versions are based on the original UNIX tools.

5) System Startup Scripts

Almost every version of UNIX and Linux comes with system initialization script but they are located in different directories:

HP-UX - /sbin/init.d
AIX - /etc/rc.d/init.d
Linux - /etc/init.d

I would like to see if anyone further adds any new points from here.

Rajesh Manne