You can download Linux off the Internet and just install it on your computer, without having to pay for it. Unlike Windows or Mac in which you have to pay for the software. You can also boot it off a USB flash drive, which is cool.
The problem is that you can't run Word or a lot of games on it without special engines or virtual machines. It's much better for programming tasks and cryptography than Windows though, and it isn't run by the Microsoft privacy-stealing panopticon.