The lower the OHM load you wire your speakers to expect a higher demand on the amp/amps. The lower the OHM = less resistance and more heat generated. Your amp and speaks need to be designed to handle the load you present to them so be sure of your equipment.
The ohm load is what the amp will be stable at and the configuration of the subs. If you want to run 2 subs at 1 ohm, you'll need two dual 4 ohm woofers. If you run one sub, you'll need a dual 2 ohm woofer to wire it to one ohm. 4 ohm is more efficient, as you go to 2 ohm and 1 ohm you get less and less efficient.