In this context it again on the essential difference between Boot Camp (which comes along with the Mac) and a "virtual machine" pointed. While Boot Camp Windows natively runs in the booting accordingly (then namely it can all resources - processor, graphics card, memory - use as if it were a Windows machine), is at Parallels Windows quasi only virtually simulated (sorry something for saloppe description). Windows will run just - as the name suggests, Parallels - parallel to OS X and takes the parts of graphics power and memory that you may refer to it.
The big advantage is then that it is actually running in parallel. One can move not only without problems data between Windows and OS X by drag and drop, but you can also take the Magic Mouse with a mop between both operating systems switch back and forth. That's really funny - and sometimes quite useful. Zickt eg the Mac browser (Flash!), You can even with a simple command to open that page in IE and Windows whoosh goes on.
That's all a breeze.
The drawback of Parallels is the decreased performance in Windows games that is really underwhelming. High end games from Schlage "FarCry" you can check off.
Those who can not do without its Windows-only programs or want, which is very well served by Parallels. The program runs with me on my iMac late 2012 without any problems and reliably. All I wanted.
Recommendation without ifs or buts.