Amplifier Power Required (RMS)RMS is normally used in hi-fi terms to describe the rated power output and is normally given with an impedance rating in ohms of the load put on to the amp when testing.
RMS is an average of all the frequencies (normally 20-20kHz) being put through an amplifier and giving its maximum recommended output level.
Because it's an average (Usually 20-20-000hz) it gives the best possible idea of what the rating is likely to be in the real world. (After all music contains a wide variety of frequency ranges, from low frequency drums to high frequency symbols)
Other ratings used by manufacturers are PMPO and DIN, RMS is always the preferred rating (Though often looks lower)