Ohm's Law: Volts X Amps = Watts. simple algebra lets you solve for whatever value you are concerned with (generally you try to limit Amps. Amps is a another term for current, ). Why is that important? So you don't have to run huge conductors to your speakers in an effort to reduce heat and burning up your wires (or the house), not to mention that they are cheaper and easier to carry around.
I would say that most Amplifiers push voltage to the speaker array at around 1/2 of your mains, example: my little Fender measures 59 volts (actual, with my Fluke Voltmeter) Incoming line voltage hovers between 115-118 VAC here in the States. Higher the voltage, the lower the Amperage for the same Wattage output.
An interesting note: Most Home Stereo speaker cabs are 16 Ohms.... which explains the dainty wires you can easily hide without much fear of setting fires or getting yourself fried if you get across them when they are powered (still not recommended).
Hope I didn't thoroughly confuse anyone yet...
"what is this quintessence of dust?" - Shakespeare