Are all power amplifiers the same? Do I need to buy different ones for different uses?
Due to clear distinctions made between the way an amplifier's output stages operate and are configured, they are not all the same. Typically, amplifiers are identified by a classification system, rating them from Class A to D. In order to read further about the differences, click here. Generally, the factors you must look out for are:

The number of channels to make sure it can fit the setup.
Power - In general, the amplifier must be able to deliver equal to twice the speaker’s continuous/programmed power rating. For example, a speaker with a nominal impedance of 8 ohm and a program power rating of 350 W should be connected to an amplifier that can produce 700 W into an 8 ohm load through a single channel.

Headroom - Defined as the difference between the normal level that an amplifier operates at and the maximum level of audio that can pass through the amplifier without distorting. Having enough headroom in an amplifier is very important to ensure that only clean and undistorted audio gets through to your speakers.

Where do power amplifiers sit in the signal chain of my PA system?
An amplifier is where the audio coming through gets amplified and sent to the speakers. It sits between the direct source of audio (mixer, interface, microphone direct to an amplifier) or processing units (effects processors, EQs, limiters, compressors, etc) and the speakers.