The GNU operating system is a completely 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 the GNU system](https://www.gnu.org/gnu/gnu-linux-faq.en.html#why)
The development of Linux is one of the most prominent examples of free and open-source software collaboration. The underlying source code may be used, modified and distributed commercially or non-commercially by anyone under the terms of its respective licenses, such
There is also Kali Linux which is used for advanced penetration testing and auditing. It has over 700 tools, and to be able to use it efficiently, you will need to know how to use the command line. Since it is 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 they occur and know what you have done, 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).
