POSIX, Linux, Unix, BSD, and GNU
*nix systems follow POSIX standards, a set of standards for implementing operating systems.
GNU is a set of rules governing free software, giving birth to open versions of existing tools. GNU/Linux combines GNU tools with the Linux kernel to form an OS, while GNU/Hurd is another variation.
Unix and BSD are older implementations of POSIX, with various levels of closed-source nature. BSD is relatively more open due to its licensing.
Linux is primarily a kernel but is commonly associated with a full OS when combined with GNU tools and other components.
Despite being POSIX-compliant, Unix, Linux, and BSD differ in their ideals and implementations.
Linux distributions are GNU/Linux distributions because Linux itself lacks a userland environment, which is provided by GNU.
BSD is a Unix-like complete OS with its own kernel and userland.
Both GNU/Linux and *BSD families (FreeBSD, OpenBSD, and NetBSD) behave like Unix-like operating systems.