[gh0sth@cker]

It is a "virtual drive". You can use this to mount images that are located on your hard drive.
So for example, you could make an image of a game onto your hard drive, then mount that image so that the image seems to be in a cd-rom as far as windows is concerned. So you wouldn't need to use your own cd and instead use the image.
It can also add emulation also.

If you go into the Alcohol options, it will let you specify how many virtual drives you want - you get to have six in the trial version and about 31 in the registered version.