Guitar amps are strange commodities in the music world. Anyone with a pickup on their guitar has got to plug into one, but it's rare that people understand just how the little boxes make your guitar sound so loud. Learning about how guitar amps work might help you learn about using the electronics and settings on both your guitar and amplifier in a more practical and efficient fashion. Here is a short lesson on how that big sound comes out of that little box.
The anatomy of a guitar amplifier is quite simple. There are just three major components – the preamp, the power amp, and the speaker. Do not be confused by those big huge stacks that people use as amps too (head and cabinet designs), they are exactly the same, just the pieces are not contained in one little box like the practice amp you have at home. Each piece of the amp serves a certain role.
The preamp is what you are actually plugging your guitar into. It receives an electronic signal from your guitar. This is also where those little controls you love to mess with are located, in the preamp you'll find the treble and bass, some fancier models have reverb and other distortion elements included as well.
After the electronic signal is received by the preamp, it is passed along to the power amp. The power amp is what super charges the signal and makes it nice and loud. If you've got a huge and powerful power amp, then you'll be able to get huge and powerful sounds from your guitar. The power amp takes the signal from the preamp (and any information about treble and bass adjustments etc.) and finally passes it to the speaker, where it reaches the human ear.
Bigger speakers allow the lower sounds of an instrument to be stronger, which is why you'll notice that bass amps are bigger than guitar amps. The most prominent part of the amplifier that you can see is the speaker, you may or may not be able to see much of the preamp and power amp. Typically all three of these components are housed inside the same unit, a little (or big) box.
With an understanding of the anatomy of an amplifier and how it works, you can now begin to experiment with different combinations. The head and cabinet design mentioned before features two parts, the head, which is the preamp, and the cabinet, which houses the power amp and the speakers. Head and cabinet designs are common for professional musicians because it gives them the ability to combine different preamps with another power amp and speakers. These are not popular among amateurs because they tend to be large and cumbersome, not easily transported from home to your buddy's garage practice location.
Understanding how guitar amps work will give you the ability to make adjustments in your sound more carefully, especially if you have a preamp with lots of features. As you progress, you'll get more particular about the sounds you want your amp to produce.