There are now several methods of to run Windows on an Intel-based Mac, and a third method still in development:
Dual booting. With a dual-boot system, Windows can directly start up the Mac. You can choose to boot either from Windows or Mac OS X, but you don't have access to both at the same time. Apple's Boot Camp gives you this.
Running Windows in a virtual machine. With this approach, you boot the Mac with Mac OS X and run Windows in a virtualization application. The advantage is that you can switch between the Mac OS X and Windows environments without rebooting. There is also the potential of moving files between the two operating systems and sharing peripherals and networking. You can do this with Parallels Desktop and iEmulator.
Running Windows apps directly in Mac OS X--without Windows. This open source project uses a Darwine, a port of the WiNE Linux program to Intel Macs. Darwine runs -- or will run -- Windows applications directly in Mac OS X. The project is in the early stages of development.
Running Windows apps remotely from a sevice. Northstar is a subscription service that hosts Windows applications on an Internet server. Mac users access Windows apps via Apple's X11.