its not true you can only download from itunes for an ipod, and you can also download from itunes for other MP3 players. You can put music on mp3 players from cd's via your computer too. With an ipod, it backs the tunes up on itunes on your computer, wheras with something like zen you can just put it straight on without storing on your computer.
When you plug an IPOD into your computer, it generally will load onto the ipod whatever songs you have in itunes (no matter if you've bought them or imported them) therefore if you delete them off your computer, they will be deleted off your ipod. Except thats not quite true because you can adjust the settings so that dosent happen
other makes are easier to understand in my opinion