some distributions are more user friendly...
basically, Linux is the kernel, the rest of the operating system (most of it) is made up of GNU software, ie open source stuff designed to be compiled on many versions of unix. On top of that, you are free to add other programs, binaries that have been compiled for your specific system, for example.
If you want to have a windows replacement, then i'd say try linux first. I'd recommend Mandrake or Red Hat, or a bunch of others but this is not the place for "what distro is best". That is totally dependent on your needs.
To download GNU/Linux or FreeBSD (a unix derivative, free and open source but based on the Berkeley Software Distribution rather than linux or GNU) go to
http://linuxiso.org/
Good luck.