are just three of the ways to run Windows on a Mac.

Boot Camp, which you already have in Snow Leopard, creates a partition on your HD into which you install a copy of Windows (XP SP2 and above). Lion and Mountain Lion require Windows 7
No problem with Mac only software, your Mac is now a Windows machine that does not know that the Mac side exists.

Parallels and VM Fusion, both of which are not free, run on top of the Mac OS. When they are installed, they create a Virtual Disk (basically a large file) on the Mac and from there you install Windows (whatever you want).

Boot Camp has the edge of Speed and the Mac actually boots up into Windows.
Parallels and VM Fusion are slower, not really suitable for game playing, but work well for the less processor intensive jobs.
Both of these will allow you to run your Mac apps, in OS X, at the same time as you are running Windows apps.
In Parallels, it is possible to blend the two systems together for a seamless melding of the two systems.
I prefer to run Parallels, Windows, in a Window of its own.

All three require that you purchase the required version of Windows.

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