What are Class A, Class B, and Class AB amplifiers?

Class A amplifiers conduct for the full 360 degrees of input cycle, offering excellent linearity but only 25-30% efficiency. Class B amplifiers use two transistors each conducting for 180 degrees, achieving 78.5% theoretical efficiency but with crossover distortion. Class AB is a compromise where each transistor conducts for more than 180 degrees, reducing crossover distortion while maintaining reasonable efficiency (50-70%).