Amps come in different classes based on their quality / efficiency.
Class D are usually used as subwoofer amps since they have relatively low quality, but are somewhat efficient... thus getting more "bang" for your buck.
There are many classes, ranging from I think A to D, including A, AB, B, BC [i think], D.
At least, this is what I read on JBL's website or something like that.
The amp works by bringing in power and AMPLIFYING it... this works with physics and a whole bunch of coily-things.
At any rate...
If you plug subs into any amp, it'll distribute the current evenly... but, this isn't as simple as it might appear. You have to compute in impedance. Check the link that brt posted. Make sure that you're doing it correctly. Don't wanna blow your amp / subs.
And always remember to set your gains CORRECTLY the first time.