Yeah, Ohm is difficult. It has to do with the load that is being placed on the amp. The lower the OHM the less resistance the amp has (edited for some crazy *** who just freaked out on me //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif ) more power the amp puts out.
An 8 ohm load is easier to run than a 4
In car audio 4 ohm is pretty much standard (although not saying you can't do others such as 8 2, 1, .5 etc).
THE AMP MUST BE STABLE TO RUN AT A CERTAIN LOAD!
Class D (usually subwoofer amps due to the frequency range that they can handle) can handle lower loads. Some go as low as .33 ohms.