Class A amplifiers keep their output devices fully on at all times. They run hot and waste enormous amounts of power as heat, but they avoid a class of distortion at the zero-crossing point and are prized by purists. Class A/B is the long-standing mainstream compromise: efficient enough to be practical, clean enough to sound excellent, which is why most great integrated amplifiers are A/B.
Class D (the "D" does not stand for "digital") switches its output devices on and off very fast and filters the result. Modern Class D — especially modules from the likes of Hypex and Purifi — measures superbly and runs cool, which is why it now appears in everything from active speakers to flagship power amps.
The honest take: in 2026 the topology tells you more about heat, size and cost than about sound quality. A well-engineered amp of any class can sound superb.