Parallels runs Windows in a virtual environment inside Mac OS X:
arallels Desktop for Mac is the leading virtualization solution for the Mac, enabling Mac users to run Windows simultaneously with their Mac OS X on any Intel-powered iMac, Mac mini, MacBook, MacBook Pro, and Mac Pro Towers.
The latest release of Parallels delivers easy to use migration tools, USB 2.0 support, innovative Coherence technology, and much more. It’s never been easier to run Mac OS and Microsoft Windows side by side.
Boot Camp allows you to dual0boot Mac OS X and Windows:
In the next major release of Mac OS X Leopard, Apple will include a new technology called Boot Camp that lets you install and run Windows on your Mac. If you have an Intel-based Mac computer and would like to try Boot Camp, you can download the public beta today.
Boot Camp lets you install Windows without moving your Mac data, though you will need to bring your own copy to the table, as Apple does not sell or support Microsoft Windows. Boot Camp will burn a CD with all the required drivers for Windows so you don’t have to scrounge around the Internet looking for them.