GNU/Linux is a free operating system that replaced the older operating system called Unix. It's ubiquitous on servers and supercomputers. It's an increasingly popular choice for desktops, but not yet common. It's also the operating system behind SteamOS.
When people say “I use Linux” they often mean a distribution of GNU/Linux, such as Debian GNU/Linux.
Unix was an older proprietary operating system from the 1970s. It's the operating system that the GNU Project was trying to replace.
Linux refers to the Linux kernel. It's a low-level component that implements syscalls, drivers, filesystems, and much more.
GNU sits above Linux kernel and it implements all of the programs and components that Unix had. Commands like ls, grep echo, cat etc. for example, are all part of the GNU package,
GNU can work without Linux (e.g. GNU Hurd) and Linux can work without GNU (e.g. Android or Alpine Linux)