Types of Licensing

We have three different kind of Licensing for Software Products.

•  Proprietary: To use this product we need to purchase license from that Organization.Source code is only visible to their development team members.
•  Freeware: They will be released as free products and End Users no need to spend any amount to use these products.Source code is only visible to their development team members.
• Open Source: Along with the end product, the source code is also available freely to download and use.We can also contribute to add any features to that product.

Unix is the Operating System released for servers first.


First Unix release is an enterprise one(Proprietary).
You need a license to purchase and use it.

Example:
HPUX (HP Unix)
Dec (IBM)
Solaris (Sun)

Later Linus Torvalds released similar code line of Unix as an Open Source version , named after him as Linux.
So, Linux is Open Source version of Unix.

Linux (Unix only but Open Source)
FreeBSD
CentOS
Ubuntu
Fedora

Redhat
Enterprise version of Linux and need to purchase it.
It is a modification/changes added version of OpenSource Linux and released separately.


The Source code of Linux is available for free at the site https://www.linux.org/

All the different flavors of Linux , like Debian, etc, are modified and customized versions from the source code available at the above link.