In any high quality home stereo you will see that there are seperate components that make up the system, the same can be said for a "Pro Audio" system as well....there are usually mixers,equalizers,crossovers and other associated items involved, however the "heart" of any pro audio set up is the power amplifier or "power amp" for short. Simply put an audio power amp is a device that takes frequencies in the human hearing range and makes them louder. The majority of power amplifiers are usually stereo which actually means that there are "two" amplifiers in a single chassis, each of these amplifiers are called "channels" ...when describing a "stereo" power amp its common to hear the term "per channel"  (as in "power per channel"), these "channels" are usually described as "Right channel" and of course "Left channel". Another characteristic of pro audio power amplifiers is that they are built to take a fair amount of abuse, they are typically used in situations where they are moved around from location to location- its not uncommon to find power amplifiers being used at outdoor shows with extreme heat, humidity and/or rain and then operating the next night inside a smokey club with faulty electrical service,  either way companies that make them understand the potential for abuse and usually have built in safegaurds to assist with the wide possibility of problems. There are many different  power amplifier manufacturers out there and of course a wide variety of prices therefore its best to comparison shop for amplifiers that share the requirements you are looking for (power, weight etc.). Power amplifiers also come in a wide variety of stated power output (wattage), I say "stated" because its common and easy for manufacturers to exaggerate the output power of an amplifier, its almost like a car manufacturer claiming thier car will easily do 130 miles per hour, its only later you find that it can only go that fast down a steep hill and only for a few seconds, technically the manufacturer didn't lie but what they claimed wasn't exactly the truth, basically the way to truthfully rate an amp is to ask how much power it can deliver from its lowest to highest frequency and with how much distortion while doing so, although a modern power amplifier can easily reproduce the frequencies of 20 to 20,000 vibrations per second (also called Hertz), it will deliver greater power into a simple "single" frequency--although a given amp may deliver 100 watts from 20-20,000 vibrations per second it may deliver 130 watts into a single frequency - this is just like comparing a person jogging while doing many things at once to a person only doing 1,of course the person doing the 1 thing will look faster and less stressed , its the same with amplifier comparison, if the power output specs written don't look something like this "20-20khz with no more then .1% distortion" and instead say something about 1 khz then you should subtract 20-30% from the amps stated power, another trick they use is to sneak in the word "peak", an amplifier should be rated using the term RMS and never peak. It's also important to find out how much power or watts the speakers you plan on using are rated for. A simple example shows that an 8 ohm speaker rated for 100 watts rms ( "RMS" in simple terms means a little above average) is usually suited to an amplifier that will send 75 to 150 watts rms into an 8 ohm load or speaker, of course this is a very simple example and dosen't go into depth about other factors. Aside from the power or wattage an amplifier is expected to deliver there are additional things to consider, one of them is the ability of the amplifier to run or "drive" more then one speaker. Lets say the speaker you are using is 8 ohms and you want to hook two of them to one channel of your power amp, the first thing you will need to know is "can this power amp safely run a 4 ohm load for each channel?"...the reason 4 ohms is mentioned instead of 8 or 16 is because when you add two 8 ohm speakers together the total drops by half--instead of 8+8 = 16 , its actually 8+8=4...  the bottom line is if you are hooking two 8 ohm speakers together make sure the amp will run 4 ohms, if you plan on hooking up three or four 8 ohm speakers together make sure the amp will run safely into a 2 ohm load- there is one amp I know of (Crest 10001) that will run safely into a 1 ohm load at full frequency response but this is almost unheard of with other manufactuers ( power amps really shouldn't run less then 2 ohms). One should also pay attention to the thickness of the wire used from the amp to the speaker, since most power amplifiers push a fair amount of electrical current its always better to use thicker gauge cable (the thicker the gauge the lower its gauge number will be)--basically there is less stress on the amp and there is more sound actually making it to the speaker. Another factor to consider when purchasing a power amplifier is weight, power amps can be very heavy and difficult to transport from place to place, its important to be aware of this when planning a purchase. As mentioned previously power amplifiers are usually very robust and last many years but lets face it, even tanks break down, I've found that power amp repair usually ranges from $150-$200, this typically includes the repair, a check up of the other components and an internal cleaning, basically its like getting a new amplifier.There are many times on Ebay you will find a non working or "as is" power amp, these are usually very inexpensive to purchase with the bulk of the price being the shipping, however when working some of these amps might fetch upwards of $1000, it may not be a bad idea to ask a qualified tech what he or she thinks about the amp you are looking at, they may have seen that particular brand or model and know it could be a simple fix.