How do amplifiers work?

It's unfortunate that magazines like Audio, or Stereo Review or High Fidelity no longer exist. In the "good ol' days" they usually ran an article on basics once a year, maybe more. In those days, we simpletons believed that there was some science behind audio that was worth knowing and considering. Then came TAS, Stereophile and the others who started spitting in the soup and telling all that whatever could be measured meant nothing. So now we have a new generation of people interested in hi-fi that cannot readily find such material. I encourage you to find out about these questions and do believe that there is one book out there that presents it cogently; if I could only remember the title. I am sure others will be able to provide it to you. The one thing that I remember is that unlike, let's say a milk bottle that can only give you its maximum quantity, an amp is not like that since it will try to put out more power than its rating when the load demands it, only with a great amount of distortion. One magazine said that it is like if instead of the milk bottle running out of milk, it started providing sour mil instead. There are many factors to consider and some others here are better equipped to explain. There used to be Sean, but he was run out of 'Agon Town on a rail for some reason that may forever remain a mystery...

My belief is that the amplifier takes electricity from the wall and stores it in capacitors (like big batteries).
The music signal (from a CD player, etc.) is used to modulate a "valve" (a tube or transistor) which results in a mirror image of the input only stonger.
Think of turning a water faucet on just a little then cranking it wide open then turning it back down, etc. This is modulation (only you are using the stored electricity not water).
This "stronger" signal is sent through your speaker cables and is heard (through your speakers) as music.

Here's how I look at it: electricity is generated by converting one form of stored energy into another. Moving water, wind, or steam made from fuel spin turbines which are like big reverse electric motors. In your home there are wall outlets. The power company regulates the energy available from them at a set standard. Using standards makes it possible to reliably run devices connected to the system.

Until you connect something that uses electricity (a load) to the outlet, nothing happens, no energy flows. Audio gear operates with its own set of standards. This allows different brands to work together. By itself, the amplifier is both a energy converter and a storage device. The amplifier has a missing part which is completed when you connect your speakers. The speakers motor electrical energy into moving air. How much air? That all depends on what qualities you want your music to have.

A CD player converts a set of digital instructions into a regulated energy flow. Likewise, a phonograph converts a set of molded plastic instructions into an energy flow. The preamplifier converts the flow into a form that the power amplifier can use. The power amplifier converts the flow into a form the speakers can use.

How well all these conversions take place is a matter of choice. The choices designers make result in performance limits. The limits of performance are conveyed to you as specifications. For example, when the conversion is out of whack (distortion) by 1% over a certain range of instructions an amplifier is said to have the capability to steadily produce so much power. Increase the distortion you're willing to accept and the amplifier can have a higher power rating. Limit the range of instructions sent to it and again the rating looks better. Don't be fooled. The important thing to remember about the power rating is that it is a description of the amp's capability, not a description of its moment to moment operating condition. An amp rated for 100 watts continuous may indeed momentarily output one or 300 watts.

An amp's capability derives from how well it converts, stores and regulates energy, how it controls heat, how well it converts its instructions, and the length of time it can be expected to do these things before it fails. Bigger is not always better. If you want to amplify music so 50,000 people in a stadium can hear it or if you want to blast your stereo at home you'll need different amplifiers to suit each application. Regardless of its specs, how subjectively well the amp fits your application is a matter of perception.