Strictly speaking, Linux is a free, open-source OS kernel originally developed by Linus Torvalds in 1991. (Linus Torvalds still oversees the kernel's development, but many people now contribute to it.) The Linux kernel needs to be combined with other software to make a working operating system, and that's what Linux distributions (distros) are. Some of the core software with which the Linux kernel is combined comes from the GNU project, which started before the Linux kernel existed, and had the goal of developing a complete free Unix-like operating system. GNU can be used with other kernels but in practice it is almost always used with the Linux kernel.

What we commonly call Linux is the Linux kernel + GNU + other bundled software to make a complete usable operating system. Importantly, this usually includes a package manager to help with installing, updating and removing software, and it often also includes a windowing system and desktop (though it doesn't have to). There are many, many ways of putting this package together, hence there are many Linux distros.

Many Linux distros prioritize free, open-source software (FOSS). Some include only FOSS while others include non-free software too.