Explain Amp classes & how effects sound?

The simplest way to think about it is in terms of effencey; Class A draws full power all the time, usually about 4 watts for every watt of rated output. So they will be large and run very hot. As you progress down the alphabet the amps become more efficient and run cooler and can be made smaller. Does this affect sound quality? Many think so but others are happy with the sound they get; only you can judge.

Thanks...so a class T amp would run cooloer than a Class A, thus a smaller chassis. I get it

In terms of heat:

class A => hottest but otherwise the most linear/lowest distortion

class A2 (tubes only) grid current may flow in the tubes during part of the waveform but the tube is conducting through the entire wave cycle. This allows for more power but has greater requirements of the driver circuit.

class AB => less heat, slightly more power but slightly more distortion

class AB2 => (tubes only) similar idea to A2, but the tubes stop conducting through part of the waveform. You get a lot of power and a lot less heat, but crossover distortion is more pronounced.

Class C => not applicable to audio

Class D => so far, for practical applications so far is transistor only. The devices are either fully on or fully off, avoiding the much greater power requirements of operating the transistors in the linear region. This makes the most power with the least amount of heat. Distortion can be very high, but this is a developing field, and is likely the area with the most potential for improvement in the next ten years.

And in follow up to Atmasphere post, Class D is the class T amp the OP is referring to.

What about class H. Soundcraftsman had some amps in the 80's class H. I had one and ran it HARD! Never got more than just a tad warm! I guess the design never took off.