This should help you get a basic understanding of Ohm's Law. Everyone who loves car audio should have this page in their favorites.
http://www.bcae1.com/spkrmlti.htm
This is an Adobe Acrobat pdf file if you want to save it to your computer.
http://www.jlaudio.com/subwoofers/pdfs/12W3_BDS.pdf
Thanks to Smarjay for giving me the updated page