Linux was started by a man named Linus Torvalds in 1991. It is open-source software which means everybody can see what's in it, and change it. If you don't like something and you know what you are doing, you can change it. Windows isn't like that. If you don't like something, you most likely won't be able to change it. There are many different versions of Linux. They are called distributions, or distros for short. Each includes the kernel, and a variety of programs chosen by the creator. Distros come in many different shapes and sizes. Some are so small they can fit on a floppy disk. Others are a couple gigabytes large. Some of the more popular distros are Slackware, Mandrake, and Red Hat. Slax is based on Slackware.
Some distros, called Live-CDs, can run from a CD, without the need to be installed to a hard drive. Some of these Live-CDs can also be run from USB Keys. Linux is very flexible, which is why so many people love it.