rms = root mean squared, that is the number you want to go buy, in an ac signal, you can do some math and determin how much real power is being provied all the time, vice peak power, which is usualy double rms power, that is the difference between 1 wave and the next, the maximum difference in potential, a lot of car amp companies will rate their products with peak power, to get larger numbers, when the rms number is what you really care about.
hope this explains it, if anyone can add anything feel free.
link
http://en.wikipedia.org/wiki/Root-mean-square
