An amplifier increases the power of a signal, and is the last device in the signal chain before the final destination – the Speakers. There has to be an amplifier somewhere, to make the signal strong enough for the speaker to produce sound.

There are 3 ways to go about this, in a PA/ Sound System environment:
A powered mixer has the amplifier built into it. Powered speakers also have internal amplification, so you would use one or the other, not both. If neither the mixer nor the speakers is powered, then there must be a separate, rack-mounted amplifier.

Powered mixers are “all-in-one” systems and, though inexpensive, they generally produce poor sound quality and fewer available options.

From Wikipedia, the free encyclopedia
Generally, an amplifier or simply amp, is a device for increasing the power of a signal by use of an external energy source.In popular use, the term usually describes an electronic amplifier, in which the input “signal” is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to make the human voice louder or play recorded music. Amplifiers may be classified according to the input (source) they are designed to amplify (such as aguitar amplifier, to perform with an electric guitar), the device they are intended to drive (such as a headphone amplifier), the frequency range of the signals (AudioIFRF, and VHF amplifiers, for example), whether they invert the signal (inverting amplifiers and non-inverting amplifiers), or the type of device used in the amplification (valve or tube amplifiers,FET amplifiers, etc.).