 Audio amplifiers use external power to convert a small signal to a higher power output. The way they do it determines the amplifier's class. Class A amps use internal transistors continuously running at full power, which results in great sound quality, but not so great power efficiency. A Class A amp converts about a quarter of its input power to sound and the rest to heat. They're pretty much reserved for the most high end of hi-fi or very low cost designs. Class D amplifiers use pulse width modulation. They output a high frequency square wave and use the amplitude of the input signal to modulate that wave's pulse width. That PWM signal then goes through a low pass filter before output. It's a much more efficient method for amplification, losing very little power to heat. The trade-off is you need more expensive circuitry and a good filter.