Linux vs BSD
Both are derivatives of Unix.
Kernel vs Full OS
Linux is just a kernel. the BSDs are both a kernel and an operating system. E.g. FreeBSD provides both the FreeBSD kernel and the FreeBSD operating system. It’s maintained as a single project. If you want to install Linux, you need to choose a Linux distro.
- Linux: GPL License. It forces them to release the code.
- BSD: BSD License. You’re free to do whatever you like with the BSD code. If you modify the BSD kernel or distribution and distribute it, you’re not obligated to release the source code.
- Darwin: the low-level kernel and other software is open-source BSD code, most of the rest of the operating system is closed-source Mac OS code.
Both are Unix descendants
Read more on the Unix Page.