What Is Linux
It's An Operating System
If you didn't know, an operating system is what allows you and the applications you run to interact with hardware. Hardware might be a computer, a phone, a tablet, a fancy coffee maker, a car, etc, etc. Some other operating systems you've probably heard of are Windows, Android, iOS, OS X, ChromeOS, and Unix. It's not used by a large percentage of the public (unless you count Android and Chrome OS, which are themselves versions of Linux) but there are still millions worldwide that use Linux on their computers. Most computer manufacturers only sell computers with Windows, so the majority of home users had to install it on their own.
Linux runs your computer, and takes the place of others like Windows or OS X.
Yes, there are versions of Linux you can pay for, but the majority of Linux distributions are free. In fact, every one I've personally ever used was free. Windows, on the other hand, was a hundred bucks a pop last I knew. And Windows comes with a pretty restrictive license. It's something like "You can only have this copy of Windows on this computer and under these circumstances blah blah blah." I'm not sure how Mac's software licenses are worded exactly, but I think that most of their stuff is proprietary. Even if it's free, there are restrictions on what you can do with it.
Linux is more like "Here. Have it. No strings attached. Give it to other people if you want."
Ideally, you're just a pee-on when you log into Linux. But that's not necesasrily bad. If your username isn't allowed to perform any kind of administrative tasks, then any malware you attract shouldn't have admin privileges either. One of the problems with Windows has been people running as an administrator, getting a virus, then the virus running rampant as an admin. On the other hand, I remember a few XP boxes where the Admin account was never used except by me (when a printer needed installing or something) and viruses STILL had the run of the mill.
On top of this, there are more eyeballs (it's open source, so ANYONE can look at it if they want to, and lots do) on the source code that makes Linux than there are on Windows or OS X code. Any security problems are fixed quicker, and flawed code is less apt to happen in the first place.
Linux, by design, is way less apt to get broken or suffer from a malware infection.