The simple difference between Unix and Linux is: Linux is completely open source, and Unix is not.
Linux is said to be a "Unix-like" operating system in that it follows the same kind of standards Unix is built on. So "Unix" or "*nix" usually just refers to a bunch of operating systems that are similar (Unix, Linux, BSD, Solaris etc).
You can install Linux on your computer by downloading one of the many many
distributions. A distro is a complete OS solution, containing the actual Linux kernel (the thing that takes care of such low-level details like memory management), software libraries, etc. There are distros for many purposes. Some are mainly used to run servers (like CentOS) and some are meant to be used as desktop OSes like Ubuntu.
Linux is Linux in the same way Windows is Windows. Think of the different distros as the difference between something like Windows XP and then Windows Server 2003 -- they contain different software and are meant for a different purpose. Just with Linux being open source, there are
a lot more options for you to choose from. Take a look at
DistroWatch for a list of distros you can choose from. Right now
Ubuntu is very popular for users looking to install Linux on their computers as a desktop OS. It's very user friendly and has a booming userbase.
All you need to do is download the ISO and burn it to a CD (Ubuntu gives away free CD's, so you can actually get them to send you a free copy if you don't want to burn one yourself). From there, just pop the CD into your CD-ROM and reboot your computer. It should boot right off the CD, and you can follow the onscreen instructions to install the OS.
Most modern Linux distors work side by side with Windows, allowing you to re-partition your harddrive so they can co-exist peacefully. Whenever you boot, you'll have an option to either boot into Windows or to boot into Linux.
As for using your iPod, I'm not sure... Someone else might have to look that one up
If you choose to install Linux on your computer, you should backup all of your files. Although it will likely go without any problems, when you're dealing with something like an OS you just never know
