Virtual AGC is a computer model of the AGC. It does not try to mimic the superficial behavioral characteristics of the AGC, but rather to model the AGC's inner workings. The result is a computer model of the AGC which is itself capable of executing the original Apollo software on (for example) a desktop PC. In computer terms, Virtual AGC is an emulator. Virtual AGC also provides an emulated AGS and (in the planning stages) an emulated LVDC. "Virtual AGC" is a catch-all term that comprises all of these.
The current version of the Virtual AGC software has been designed to work in Linux, in Windows XP/Vista/7, and in Mac OS X 10.3 or later (but 10.5 or later is best). It also works in at least some versions of FreeBSD. However, since I personally work in Linux, I have the most confidence in the Linux version.