Linux is an operating system - the basic software that makes a computer function, or the same sort of thing that Windows, OS X, and Android are. In order to maximize its users' control over the operating system, nearly everything to do with the OS's development is open-source (like Mastodon), completely transparent, and interoperates with other programs. This makes Linux great for specialized uses, so it's the dominant operating system for pretty much everything except personal computers and smartphones.

There's no single entity developing Linux; rather, it's pieced together from a lot of different programs created by different people with similar goals. Some of these programs have overlapping uses, and as such Linux can be pieced together in numerous different ways, creating different versions called "distributions" or "distros." The only program every Linux distribution must have is that way on a technicality - the Linux kernel, because it's the namesake of Linux. You pretty much have to make your own choices at every level of the operating system, which is great for users' freedom and control, but not as good for ease of use - and that's the root of pretty much everything good and everything bad about Linux.

Sorry if this is long, but this is what you get for asking an autistic person about his special interest. If you need clarification, please feel free to reply to this or DM me on Mastodon.