PA stands for public address and is by definition technical equipment for musical events. A PA system is a functional interaction of several components. First off this includes various speakers that have their own individual spectrum of tasks and frequency ranges. The speakers are energised by power amplifiers and the sound and track settings are controlled by mixer consoles.
Instrumentation amplifiers, drums and voices are miked with microphones and fed to the mixer console. In order to optimize the live sound, particular effects like reverb or limiters are used. Commonly those effects are looped in or added directly to the master signal. Audio crossovers separate frequencies and assign them to the respective speakers.
Dedicated speakers called monitors give musicians the ability to retain control over their own sound. Furthermore various accessories like cables, cases, racks – that e.g. contain effects or amplifiers – are needed for any successful event. In case of large events communication tools like headsets are used to communicate between stage and FoH (Front of House).