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 as the GNU General Public License. The primary difference between Linux and many other popular contemporary operating systems is that the Linux kernel and other components are free and open-source software.

Bellow, a very good Introduction from Elie the Computer Guy.

Topics Covered
What is Linux
Distributions
Open Source Licensing
Shell
Significance of ROOT
Importance of Capitalization
Server vs. Desktop versions
What Linux is best at
Linux is not a version of Unix.