The OS of choice for the raspberry pi it runs on almost everything, from desktops to laptops.
Whilst Windows achieved success it did so through a monopoly, it usually is only good at communicating with itself.
Linux tries to communicate all protocols on all devices. It's the biggest project the internet has ever undertaken
It runs on 80% of all web servers as well as most super computers.

It's efficient: A standard install with LibreOffice, firefox, MP3 Player and Video player takes up 2.8G Gigabytes of hard drive space,
This is considerably less than the space occupied by windows, even less than the 3GB download size of Office 365 alone.

It's free in one of two ways: developers are not allowed to charge money for the software which uses it (they get revenue in other ways).
We know how it's made, most of the code for it is publically available, this means that anyone can take a project and improve on it.
Free stuff usually seems a bit naff, or limited. This really is not.

Android is derived from it. It shares a lot in common with Mac OSX, but it's free. You can do what you want with it,
from tinkering to seeing all parts of what it's up to.

Started as niche uni project by a Finnish man called Linus Torvalds it's becoming easier to use over time, though it can get a bit complicated.