On your lovely Mac, when you plug it in via USB, or firewire, an icon will appear on your desktop, with whatever name your drive has, thats it. dead easy.
If you have more than 1 computer, I would strongly recommend something more like NAS (Network Attached Storage) This
is a Linksys SLUG which is a network controller for USB storage (which will allow you to attach multiple USB disks to your network - I am paranoid about data loss).
Anyway, the LaCie USB external drives are excellent, and designed primarily for Macs, so I would recommend them, even though they aren't the cheapest.