It's another operating system for computers, and in most cases, it's free. When I say "those who can" is because you need to know how to operate it, which... hmmm... they're a lot of versions, and not all are the same.
So take Ubuntu, a Linux distrobution (aka operating system/distro). It's probably the most commonly used version, and very similar to windows but with its own twist. If you know the subtle differences, it's no big deal, but if you don't, good freaking luck.
Linux is actually super old, and I do believe precedes both Macintosh and Windows. It is however an open source code which is something neither IOS or WIN will ever be. Basically, you can use the existing architecture of one Linux distro, and totally customize it to your exact wants, assuming you know how to. Again, completely free. It's also much easier on your general system (within reason) as it's not has resource hungry.
So... Those who can't, IOS/PC.