well said...^
let me put it in words i understand....
when dealing with car audio....OHM is a rating on the speakers, per voice coil. and on the amp, the amp gives out different amounts of power at different OHM. the lower the OHM, the more power it can give off. but some amps wont play at too low of an impedance (OHM) because it will cause them to run really hot.
so what you will have to do when looking at sub and amp combos, is the impedance of the sub, and the what impedance does the amp give the most power. if they match...cool
RMS.....thats the power rating of the speaker. peak doesnt mean much...RMS is a number to look at.