VMware Fusion and Parallels DesktopSEE IT are the two apps that make it easy to run Windows apps on the Mac OS X desktop, and they're so similar in features and functions that it's impossible to say which one you should choose. If you're a Mac user who needs to use an app that runs only under Windows, then you definitely need one of them. But you won't be able to choose on the basis of which one is more up-to-date, because both came out with shiny new versions within a few days of each other.

Both let you run Windows 8.1 on the OS X desktop; both run under OS X 10.9 Mavericks—the next version of OS X—and both let you run a copy of OS X 10.9 or other recent OS X versions in a window on whatever version of OS X you have running on your Mac. I slightly prefer VMware Fusion, for reasons I'll describe in a moment, but I have both of them installed, and I'm very impressed with Parallels Desktop's ease of use and suitability for beginners who may find VMware Fusion slightly daunting.

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By the way, I wrote that VMware Fusion and Parallels are the two apps that "make it easy" to run Windows apps on the OS X desktop. You can find other apps that do far less and require far more work, notably the open-source Oracle VM VirtualBox and various free and commercial versions of the WINE (WINE Is Not an Emulator) framework. None of these come even remotely close to VMware or Parallels in power, speed, or convenience. You can also run Windows on any recent Mac using Apple's own Boot Camp technology, but Boot Camp makes you shut down OS X and restart in Windows, then shut down Windows and restart in OS X, while VMware and Parallels let you run (for example) Microsoft's new Windows-based Office 2013 in an OS X window while running Mac-based software in other windows—and all these apps get direct access to any OS X folder you choose.

Installing an OS
VMware Fusion, like Parallels, starts up with a wizard that lets you install a "guest" OS to run inside your "host" OS X system. Unlike Parallels, which gives you a smorgasbord of free OSes to choose from (Android, Linux, Windows 8.1 Preview), VMware expects you to do one of these things: install an OS from an installer CD, DVD, or disk image file; import an image of your existing Windows machine using either a network connection or a direct connection via Ethernet cable; install a second copy of OS X from the recovery partition hidden on any Mac running Lion or later; or use a Windows system that you've previously installed via Apple's Boot Camp. If you choose an operating system that VMware supports—any recent variety of Linux, Windows, OS X and more—then VMware will install it and launch it automatically. More arcane operating systems require you to enter options by hand as they start up, just as on real hardware.

Parallels offers prebuilt options like Android from a menu, but you can find a far vaster range of prebuilt VMware machines if you're willing to search for them on VMware's site and elsewhere. (Search for "VMware appliances.") Programmers have even written VMware drivers for arcane operating systems like NeXTSTEP and OPENSTEP, the OSes developed by Steve Jobs that later evolved into OS X. Purely for curiosity's sake, I have both those ancient OSes running in VMware machines that I found in the depths of the Internet. But I use VMware more often to run Windows apps that don't exist on my Mac—for example, Corel WordPerfect X6 and Axialis Icon Workshop, which is, ironically enough, by far the best editing tool for OS X icons, and it exists only in a Windows version.