You just had to ask, didn't you. Linux is a kernel (never mind what that means, it doesn't matter). It uses the Gnu utilities and there are at least a hundred different versions or flavours available. Most of them are free. Go to distrowatch.com to get some idea whats available. For the the utter beginner a distro called Linspire or Freespire might be a good choice. Ubuntu, last time I heard, will send you a free installation cd. The 2 biggest drawbacks from your probable point of view are: the bound and printed documentation either stinks or doesnt exist at all and - of course - it's not windows, so things are done differently, sometimes in ways you might not expect. There are on-line communities for every distro, often very helpful. If you're the kind of person who explores the operating system just for the hell of it you may like it.