Conceptually Linux and Unix are the same. The main difference I see is that Linux has evolved into an attempt to replace Windows on the desktop that also works quite well as a server and Unix generally isn't a replacement for a desktop.
So Linux has a bunch of User type stuff. mp3 players, graphics programs, cute window managers (which I like, don't get me wrong) and people who are converting from Windows to Linux. So things like wireless card drivers, graphic drivers, and device like USB stuff are important.
Unix (as in Solaris, AIX, HP-UX) is more a server side os, used for web servers, e-mail, and other servers. While they are used as workstations, they're more likely used for scientific or high-end graphics processing than as a replacement for Windows to do web browsing or reading of e-mail.
If you're downloading Linux, you can find anything you want over here:
http://distrowatch.com/
You can retrieve a Live CD which is Linux on a bootable CD or DVD so it doesn't mess with your current installation or you can download an image you can use to install on your system. Ubuntu seems to be a pretty good distribution nowadays.
Since you're downloading Linux, I'd hunt for a beginning Linux book since examples and such will be available on your installation.
Carl
