It's the difference between continuous power output and dynamic output. For example, the used Yamaha M80 2 channel amp I just picked up for a song is rated as follows:

Continuous Power Per Channel

20Hz-20KHz 0.003% THD 8 ohms ...... 250 Watts
20Hz-20KHz 0.007% THD 6 ohms ...... 290 Watts
20Hz-20KHz 0.020% THD 2 ohms ...... 330 Watts

Dynamic Power

1 KHz 8/6/4/2 ohms .................. 380/480/640/850


Enjoy the Music. Trust your ears. Laugh at Folks Who Claim to Know it All.