The GNU operating system is a complete free software system, upward-compatible with Unix. GNU stands for “GNU's Not Unix”.
The project to develop the GNU system is called the “GNU Project”. The GNU Project was conceived in 1983 as a way of bringing back the cooperative spirit that prevailed in the computing community in earlier days—to make cooperation possible once again by removing the obstacles to cooperation imposed by the owners of proprietary software. *The GNU operating system and the Linux kernel are separate software projects that do complementary jobs. Typically they are packaged in a GNU/Linux distribution, and used together*. More at [Overview of GNU system](https://www.gnu.org/gnu/gnu-linux-faq.en.html#why)
There is also Kali Linux which is used for advanced penetration testing and auditing. It has over 700 tools, to be able to use itefficiently you will need to know how to use command line. Since, it used for ethical hacking and pen testing you will have to perform certain malicious hacks which might brick your system. Therefore, one should have complete knowledge about the command line interface, so, that you can fix errors if occured and knew what you have done and hence it is not advised for beginners.
Linux distributions also come in all shapes and size and most if not all offer the ability to run directly via CD / DVD in what's known
as a 'Live CD' environment or even directly via USB if your motherboard supports booting from USB.
For more facts about Linux, read Quincy Larson's article [Linux is 25. Yay! Let’s celebrate with 25 stunning facts about Linux](https://medium.freecodecamp.org/linux-is-25-yay-lets-celebrate-with-25-rad-facts-about-linux-c8d8ac30076d).
