How Does A Power Amplifier Amplify Power?

Amplifier is a device that is designed to take in a low level signal and make it stronger; technically speaking it increases its amplitude. Amplifiers can be seen in every day life for instance in many audio application. Power amplifiers do the same. But be careful here, Power is not some thing that can be amplified therefore we must be clear that when we talk about power amplification, it is either the voltage or the current that gets increased or amplified. For instance imagine a CD player being connected to the speakers in order to amplify the small signal from the CD player. In most cases the input signals voltage is increased using the AC wall outlet to which they are plugged into, and hooking a transistor such that when the load remains the same and high current passes through the load. This can be explained in the simple equation of V=IR. When this happens power eventually get amplified as P=V*I, where P=Power, V=voltage, I=current and R=resistance as in load or speaker.
Every amplifier contains a transistor for the purpose of amplification and it is the transistor if designed in perfect manner can do the amplification job. In our example, a common collector or an emitter follower meaning that collector node of the transistor is tied to a power rail or the common node is used to amplify the power signal. There are many formations for the transistors and one can learn using modern Electronics books.
They are many classes of Power Amplifiers too. Some famous ones are Class A, B, AB, C and D. Each of the amplifiers are classified according to their efficiency in converting the AC wall outlet power as discussed before to output power.