HTG Explains: What’s the Difference Between Linux and BSD?
Both Linux and the BSDs are free and open-source, Unix-like operating systems. They even use much of the same software — these operating systems have more things in common than they do differences. So why do they all exist?
There are more differences than we can cover here, especially philosophical differences about the way one should build an operating system and license it. This should help you understand the basics, though.