Linux is an open source operating system.  It is a lot like Windows or Mac, but not quite as user friendly.  It is user friendly as far as just surfing the web, using office programs, or listening to music, but, not so much for installing programs or setting up.  Linux does not run Windows programs natively, but you can run some Windows programs through an emulator called Wine.  One advantage of Linux is, like Apple's Mac, it doesn't get viruses.  Another advantage of Linux is that it is free, unlike Windows or Mac.  Linux is also very stable.  If set up correctly, you can leave it running all of the time, without it crashing or having to restart it.  You can also dual boot your computer with both Linux and Windows.  What that means is you can have both operating systems installed on your computer, and choose which one you want to use when you start up your computer.  Some versions of Linux will even run directly off of a CD.  This allows you to try it out without installing it or making any changes to your computer.  It runs kind of slow this way, compared to having it installed, but it's a great way to test it out.