There are amps out there that produce the same power at 2, 3, and 4 ohm impedance. Not to mention the amps that are not stable at a certain load. You can wire your subs to a certain impedance that you want to present to the amp. Different subs have different impedances, so you will wire the sub(s) differently depending on the impedance of the sub(s) and what the amp is rated for. If you want a two ohm load on your amp (say your amp puts maximum power out at 2 ohms) you will wire your sub(s) in such a way to present a 2 ohm load on your amp. Now, depending on the subs, this is sometimes impossible. Thats why you must carefully plan out your system before you buy it, you could buy an 8 ohm sub and an amp that puts out maximum power at 2 ohms, and you would be screwed because the minimum impedance you can send to your amp would be 8 ohms.

This is a basic rundown on impedance- to learn more go to www.bcae1.com and read up on series/parallel basics and series/parallel impedance.