A live sound reproduction system has two main forms:

A "sound reinforcement" system enhances the volume of the initial sound and will be designed so that as much as possible the listener will not realise that an artificial system is being used to make it easier for them to hear the initial sound.

Sound Reinforcement is also the name applied to an entire industry that involves manufacturers, "Sound Engineers" and "Sound Companies" If you see a live concert, these people are usually involved. What they do is considered "sound Reinforcement" but contrary to the above mentioned definition it is very obvious, Especially for poular music where the PA system is very obvious and generally all you hear and feel.

A "PA" (an acronym of "Public Address") is a usually simpler design that just amplifies the initial sound and it will be obvious to the user that it is being used. PA systems require a microphone, an amplifier of some kind and a loudspeaker and can range in size and complexity between handheld (megaphone) or requiring several large trucks to move.