A power amplifier is much like the recent TV advert, in that it pretty much does what it says on the tin.
It is an amplifier that provides amplification power to a mixer and/or PA system. Generally this type of amplification uses solid state technology to produce pristine sound at high volumes without the merest hint of distortion and indeed that is the primary role of power amplifiers.
Amplifiers come with their own particular jargon and you may see amplifiers described as "Class A" "Class B" and so forth, this basically refers to the design of the circuitry within the amp, as an example Class A amplifiers have very low distortion and are therefore ideal as power amps.
Heavy Duty Stage Equipment
One thing you do need to be aware of with power amps is that generally they are very heavy and therefore you need to be careful when lifting them - you have been warned. The power amp is normally connected to a mixing desk via connecting cables and the speakers also connect into the amp via a similar method.