Amplifiers  
Tech bit As the name suggests, a four-channel amp has just that - four channel to power the speakers. Two channels power the left-hand side, and two do the right. Some amps even have the option to a front and rear split, which allows you greater control over the sound. This is also known as sound staging and gives you greater flexibility over your whole system. There are a number of ways you can wire these amps - they can run in four,  three or two-channel modes.

Different amp modes
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Four-channel amp mode
Simple. The amp runs four speakers, each allocated to its own dedicated channel.

Three-channel amp mode
Running in three-channel mode, also known as tri-mode, involves something called 'bridging'. Bridging two channels creates a single, more powerful channel. Use the positive connection from one and the negative from another, and you end up with one channel. This is usually one to one-and-a-half more powerful than a normal single channel. This is ideal for a sub as it needs more power. By doing this you still have two of the four single channels left to run a set of speakers, so it's the ideal set-up for a system comprising component speakers and a subwoofer.
Two-channel amp mode
This is were you bridge two sets of channels, creating two powerful channels. This is great for running large speakers such as 6x9s, 7x10s and subwoofers that need the extra power. Not all four-channel amps are able to run in two-channel mode. It's not vital, but could be useful when you upgrade your system in the future.

Crossovers
Built-in crossovers allow you to tweak each channel and filter out unwanted high or low frequencies. By adjusting these you can send bass, mid-bass, midrange and treble to the right speakers, then tweak the channels with the corssover. In the future, as you system grows, you can calibrate the amp to run a set of midrange speakers and tweeters on their own channels, without the need for an external crossover. This not only gives you more power, but more control over the sound.

Amp gibberisch explained
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HPF
High-Pass Filter. This filter cancels out the really low frequencies for both mids and tweeters.

LPF
Low-Pass Filter. This cancels out the mids and highs, and sends only low frequencies to the subwoofer.

MOSFET
Metal Oxide Semiconductor Field Effect Transisitors. Used in the amp's power supply, this type of transistor runs much cooler, and all quality amps now use them.

Install tips
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Cooling
Amps need a lot of cooling. This is provided by the metal cover known as heatsink. These come in all sorts of designs and some incorporate a fan to aid cooling. Think about where you plan to mount the amp. If it gets too hot, it'll fry. If you plan to mount it behind a panel, fit a fan separately if it doesn't have one. Never mount an amp upside down unless the manufacturer say it's safe - you might just direct heat through the circuit board.

Power
Amps can need extra juice with big bass systems, so think about fitting a power cap - it'll store extra power and release it just when it needs a boost.

Controllers
Dash-mounted bass boost controllers let you control the bass from the front - handy if your Mum's in the car or you've got a hangover.