They are virtual machine applications. They act like a completely independent computer inside the window. Thus you can install unix, dos, windows, and even in some cases MacOS. The operating system is running in a virtual machine and thus you can do a lot with it and it won't corrupt your main operating system (MacOS). But because they are emulated, they are slower than native operating systems running directly on the hardware. This means that in general they aren't very good for gaming (not that you can't do it, some people do, especially older games).
I use mine every day for work. We have about a dozen or so different windows configurations with another half dozen or so applications for each. It gives us the flexibility to test our hardware with different operating systems. I use it for my windows development; and thus my development environment can be completely independent from my test machine. In fact, by just copying over the file(s) for the test machine I can always run on a clean version of windows.
For every day use I have a Development machine, a Office Machine, a Synthesis Machine, a Test machine, all sitting on the desktop of OSX.
From a company stand point we have moved to requiring every project, when it is started, installed / developed on a clean virtual machine. The developer can install their tools, their environment, and when the project is done, it is added to our projects repository. This way if someone later on has to go back to make changes, all of the tools are already installed, all of the versions of the tools are exactly what the developer used, and the build environment is all setup exactly as the developer left it. Often we can distribute these environments with our hardware to our customers (we do a lot of contract work). This way, they get everything left in our last state and we know works perfectly.
Hope that helps.
BTW, if you want to just play games, yes, you can setup boot camp and reboot your computer into windows, that will give you the best performance anyway. But it does require you to reboot.
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