Virtual DJ for Mac is a virtual music-mixing program, the capabilities of which vary greatly depending on what version you're paying for -- from the free "Home" edition on up to the $299 "Virtual DJ Pro Full."
That is to say that Virtual DJ for Mac is serious software, and experienced turntablists and club DJs will almost certainly want to check out one of the more full-featured packages, which offer extras like practically unlimited decks (up to 99), full-screen video output, and support for MIDI/HID controllers. For DJing neophytes, being plunged into the overwhelming Virtual DJ interface is like learning how to drive by being dropped from a helicopter onto a moving motorcycle -- i.e., it'll be really, really fun for a while, but it probably won't be pretty. We strongly recommend Virtual DJ's online forums, wikis, and manuals, which offer extensive information not just on the Virtual DJ product but also on DJing basics. If you're trying to learn how to DJ, the Home Free version puts a ton of tools at your disposal -- from two-deck mixing and instant BPM detection to built-in samples, looping, effects, recording, and more -- all integrated into the music that's already on your computer. It can be a lot of fun to play with, and aspiring DJs may find themselves working their way up to Virtual DJ's more all-pro packages.