It's basically an amp, but doesn't require the direct line from the battery. I think the advertised term is a power pack. I had it on my Commodore and it boosted the output of the head deck, and it just ran off the stereo loom. Once I upgraded stereos though, it wasn't needed.
I had another look today, pulled all the dash apart, glove box etc, and haven't found the factory amp others are claiming should be there, so no idea. I have removed the power pack/booster for the time being, but would like to reinstall when the speakers come, as the head unit isn't going to be strong enough to drive them. You have to crank it with the stock speakers to hear anything.
It sounds kind of dumb, I know, but would like to keep the factory stereo. Have looked at facia kits and what not, and due to the center dash design, they look quite ugly, or are cheap eBay brands.