A given amp will produce a higher wattage at lower ohm levels. This is why you will see a number of figures listed for each amp. Most car speakers are 4 ohms, so you can base your comparisons on that. Subs, on the other hand, can come in different ohm ratings, and you will want to use the lowest ohm rating that your amp allows. Subs can also be wired in parallel or series to affect the resistance, where stereo speakers can't.

Lower Ohms = More Resistance = Higher Voltage = More Work for the Amp

Matthew
PS: This is rather skimpy overview, i'm sure someone else will explain better.