Unix OS was developed by Bell research. Now, they are commercially available.
Unix supports POSIX  interface.
MAC OS X for example is a UNIX operating system.
However, most UNIX operating systems are considered as a complete operating system as everything come from a single source or vendor. That’s why they are commercial.
Unix is a proprietary software operating system. As said above, it was developed by Bell Labs for its own use but over the years was licensed to other tech companies. The Unix OS works primarily on Command Line Interface, though, recently, there have been developments for GUI on Unix systems.
Unix is not free. Different flavors of Unix have different cost structures according to vendors.
Unix is not as flexible as Linux. It has less compatibility with different types of hardware. Unix installation requires a strict and well-defined hardware machinery and works only on specific CPU machines.
Since Unix OS is not a freeware and therefore its source code is not available. You cannot modify and sell your version of Unix therefore.
Linus from helsinki Developed Linux OS, which is open source. Now, it has many distribution.
All Linux Kernel (which is OS) includes a GUI, GNU utilities (cp, mv, ls,date, bash etc), Package installation and management tools (apt for Debian/Ubuntu, Yum for Centos etc),
Editor (vi), GNU c/c++ Compilers and various applications.
But, the software package can come from different provider.
In Linux distributions also, there are some commercially backed distro. And some are free.
UNIX Operating System Names
A few popular names:
- IBM AIX
- Sun Solairs
- Mac OS X
Linux Distribution (Operating System) Names
A few popular names:
- Redhat Enterprise Linux
- Fedora Linux
- Debian Linux
- Suse Enterprise Linux
- Ubuntu Linux
I will also list some distros with their popular uses. 
- Fedora: Cutting Edge Technology Implementation
- RedHat and Debian Server
- Ubuntu: one of the Introductory distro for Newbies
- Kali and Backtrack: Penetration Testing, etc.
Now, RedHat, Fedora, Centos all from Red Hat Enterprise Limited (RHEL).
RedHat, Fedora, Centos uses Yum package manager.
- RedHat is commercial. (Official developers) Popular for servers.
- Fedora is Commercial. It is well known for its bleeding edge technology implementation, which is released every six month.
- Centos is free. It is RedHat minus non-free packages. It is also very stable. latest version of packages are pushed to Yum after Testing. The focus is stability.
Why people buy linux? Because RedHat is stable.
Debian on the other hand is a Linux distribution which is very much stable and contains very large number of packages into its repository. Any other distribution that comes close to Debian at this point is Gentoo. On my Debian server (Squeeze), which is a bit outdated.
Debian is another stable Distro. It has very large number of packages. Nearly 37000.
You see packages more than 37.5K! Everything you need is present in the repository itself. The package manager Apt is too smart to resolve all the dependency problem itself. Very rarely a Debian user requires to download and install dependency manually. Debian is built with a number of package manager which makes package management a cake walk.
Ubuntu which is a Linux distribution for newbies. A newbie Linux Enthusiast is suggested to start with Ubuntu in most of the Linux forum. Ubuntu maintains a simple and user-friendly interface, which gives a feeling of Windows like OS to a new user.
Debian is the base of Ubuntu, but their repository varies. Ubuntu contains newer updated packages and is still stable. In-fact Ubuntu is highly appreciated by newbies as well as advanced users.
Taking the above description into the next stage by presenting them in a point-wise fashion for better understanding and reference, here we go.
1. RedHat is Most Widely used Distribution for servers.
Debian is widely used Distribution next to RedHat.
2. RedHat is Commercial Linux Distribution.
Debian is Non-commercial Linux Distribution.
3. RedHat contains roughly 3000 packages.
Latest Debian Release (Wheezy) contains well over 38000 packages.
4. RedHat bug fixing takes considerable time, since it is controlled by a small group of people-RedHat Employee.
Bug fixing in Debian is very much quick as people all around the globe from Debian community, working from different geographical location simultaneously fixes it.
5. RedHat don’t release package updates, till next release, means you have to wait for the next release be it minor.
Debian community believes – software is a continuous evolution process, hence updates are released on Daily Basis.
- The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines the application programming interface (API), along with command line shells and utility interfaces, for software compatibility with variants of Unix and other operating systems.