AMPLIFIERS COME IN 3 GUISES:
Pre-Amplifier: Controls volume and source selections. To be used with a seperate power-amplifier
Power Amplifier: Provides the power. To be used with a seperate pre-amplifier
Integrated Amplifier: Comprises both pre & power amplifier in one component.
There are different amplification methods, these are the most popular:
Class A - inefficient but Great sound quality. It's like an incandecent light bulb, it produces more heat than power.
Class A/B - A more efficient version of Class A but suffers from crossover distortion. It's a mature technology and the best brands manage this Achilles heel well.
Class D - Best possible efficiency. Sound quality has improved tremendously in recent years. It's like an LED light bulb and produces more power than heat.
Class G - a more efficient version of Class A/B.