Linux is the name of the kernel (drivers and core libraries) that is used by all Linux based distributions. Linux based distributions are Unix-like and generally open source operating systems which run on open source software.

There are 300+ different Linux-bases Distributions available, most of which are free. The non-free (commercial) distributions include RedHat, Suse and a few others. In most cases with free distributions you can visit the product's website and download the CD/DVD image and freely install the OS onto your computer. You can get a fairly good list of Linux Distros are www.distrowatch.com

Linux and Unix based operating systems are functionally different from window do to different core structures, libraries and application sets. In addition nearly all of the software and drivers are open source which means that they have been developed by a group of volunteers which are using open source licenses that allow the users to freely customize or reprogram any aspect of the software.

@srvtk

I'll keep it brief and give you the systems I prefer for servers.

1) debian squeeze
2)ubuntu server 10.10
3)centOS
4)fedora

These distros are freely downloaded from www.linux.com, www.distrowatch.com and from the distros' website. I like the debain/ubuntu based systems for desktop and servers because they incorporate methods for easy software installation and system administration. If you want a system for testing, I prefer to use, Debian core, slackware, or Archlinux. Happy hunting.