Power amplifier modules are compact, individually installed units that increase the power levels of signals to a large degree. An electronic loudspeaker module that produces sound that can be heard from far away is just one example.
"Amplifier" is a generic term for any device that increases the strength of an input signal. Amplifiers are extensively used in electronics to increase the strength of everything from sound waves to electronic currents. Power amplifiers specifically increase signal power. They are often the final amplification stage in a device, lending the signal one last boost to make it suitable as output. They are especially used in audio applications, such as stereo systems or loudspeakers.
Power amplifier modules can be swapped in or out of devices, as they are individualized units. Some can even be used as standalone devices, while others must be installed into electronic systems to work.