Previously, there were only 2 ways of running Windows on a Mac:
- Using Apple's boot camp (which installs Windows natively on the machine)
- Using Parallels Desktop for Mac, which is a VMWare competitor and beat them to the punch in stitching up Mac users who wanted to run a virtualised environment.
Oh, the other thing is that you have to pay for Parallels. VMWare has a free version of their product, and we all LOVE free (although I'm not sure if VMWare intend on charging for the Mac version once it's out of beta).