The idea is that you still use Mac OS X as your primary OS (it's better than windows in many respects), but often you'll say be a web designer that wants to see what your website look like in IE6 on windows. so you can boot into windows and check it out. do whatever you need to do, then back to your usual environment of OS X.
Virtualisation is also being noticed too: Parallels software for example lets you run windows at the same time as OS X (and not having to boot in and out). and it's rumoured that Apple themselves may be building a virtualisation layer into OS X itself for the Leopard release, probably around August 7th.