RMS stands for "Root Mean Square", which is fancy mathematical formula for finding the average amount of power an amplifier can continuously produce. There is no legal standard for calculating RMS watts for an amplifier. Most amplifier makers get a "UL" Rating from the Underwriters Labratories to obtain the most 'true' RMS rating.
Since most amps sound their best when they are 'cranked', some musicians look at the Peak wattage more than the RMS rating. Also, tube-amplifiers are usually not rated by RMS.
IM not answering my own question here am I ?
Underwriters Laboratories (UL) doesn't rate equipment for anything but safety. The RMS ratings for amplifiers, and the testing procedures specified for deriving these ratings, come from the Federal Trade Commission.
True story: in the early 1970s, when the stereo craze was at full tilt but before the FTC had stepped in to mandate a level playing field, amplifiers were rated at "peak" power - usually into a 1 or 2 ohm load. Seeing an audio amplifier not much larger than one of today's HTs rated at 1500 watts "peak" (meaning that the reading was only microseconds' duration) was just ludicrous. I was visiting a local Radio Shack and noticed lots of stereo equipment with new signs advertising "RMS". I decided to play dumb, and asked one of the clerks about it. "Ah yes!" he explained. "That stands for 'Real Music Sound'".