This allows you to run a "Virtual Windows Machine" on the system while booted into Mac OS X. This is the method most often preferred by Apple users to run Windows applications as it does not require the user to reboot into Windows in order to use the software. The virtual machine runs inside of a window inside of OS X.
What you need:
§ A copy of Microsoft Windows and Valid License/CD Key
§ Some sort of virtualization software such as Sun VirtualBox, VMware Fusion, or Parallels Desktop
§ A system powerful enough to support the virtualization of the version of MS Windows which you have chosen