There is a really good site that explains all that but the link got deleted from my bookmarks. I am sure someone will come along and post it //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
ohm is a unit of electrical resistance. subs come in single voice coils, dual voice coils, quad voice coils..
the number that follows it tells you what ohm each coil is..such as a single 4ohm sub wires to 4 ohm. a dual 4 ohm sub gives you the options of wiring to 2 or ...something not useful. so you need to look at the sub and find what coils it has so that the amp can match up so that its peak power is actually being used.
ohms is a measure of resistance...........higher ohm rating more resistance, one ohm rating is not better then another, it all depends on what your amp is rated for, generally you can get more power at a lower impedance. so if you have a dual 4 ohm subwoofer you can wire it in parallel down to a 2 ohm load and get more power out of your amp