What does a pre-amp do?

I am some what of a beginner in the hometheater world.I am looking to tweak my system some more but I dont know what some components do.My hometheater consists of the following.
Mitsubishi ws809 55" widescreen rp. tv
Denon AVR3802 reciever
Toshiba 5700 dvd with progressive scan
SLV-N81 sony vcr
Digital cable box
Boston Acoustic PV 600 Sub.
Boston VR 910 Center
Boston VR 950 Front right left Towers
Boston Micro 90 Surround right and left
Bose Rear center channel
So as you can see I could use some tweaks so I am trying to find out what a pre-amp. does.Any comments or recommendations would be greatly appreciated.
A preamp is basically the part of your receiver that controls the volume , bass , treble and switching from one component to another . It handles the signals coming from the source unit(cd,dvd,tuner, turntable) before it gets to the amplifier. A receiver contains a tuner, preamp and amplifier - in the case of an av receiver maybe mutiple channel amps. In separates the preamp is a total stand alone unit without amp. It may or may not have a tuner (usually not) . In some cases the source (cd player) has a volume control and you can bypass having a preamp and thus a direct connection to the amp- usually better sound . Your preamp is part of your Denon receiver and for your equipment and setup you do not need another preamp.
You could however use your 3802 as a pre-amp itself. This could be done by hooking up one or more amplifiers to your reciever through the pre-amp outs, (instructions in your manual or help from your dealer). This will defer the use of your reciever's amps to the external amp(s), giving you better sound by seperating the electronic circuits.
Jondon: I did not fly spek your system but as noted above by Ljgj a pre amp is probably not the first way to go for a tweak in your system.

I will say one thing more about a preamp though if for future reference only.

An audio system does work. Electrical work. They refer to the work current has to do sometimes as a "load." Among other things, all of your components present a certain kind of work-load to each other. A certain amount and kind of work. Often this is refered to as impedance. Your cd player has an output impedance and your receiver, or a pre amp if you bought one, would have an input impedance. One important thing a pre amp does is to match the source's output impedance to the amp's (cd,phono,or whatever source) input impedance.

If your cd has the right impedance you may be able to plug it into an amp with good results. Often however this is not the case. And, if you have several sources the odds are that they will not match your amp and you will need a pre amp to do so. This is really a simplification because the ability of your pre to deliver current (slew rate) and it's design (does it have large output coupling capacitor) also influence it's ability to do electrical work.

Sometimes there is a figure that the ratio must be at least 1 to 10 or up to 1 to 100. That is, if your preamp's output impedance, if 800 ohms, it should be matched with an amp having a least 8000 ohm input impedance and up to 80,000(they call impedance "Z"). You here this all the time and, in my opinion, it is simply wrong. If you make the wrong match here you will not be happy and spend all sort of $$$ on cables or something when that is not the trouble.

I will give you a rather lame analogy to help you think about impedance. Impedance is just a form of resistance needed to get work done. If you flap your arms you are not going to fly. The "load", your body, is too heavy and the air is not DENSE enough to push against. You can really spend a lot of effort and get nowhere. The air's impedance is too low. There is an impedance mismatch here and so we cannot fly by waving our arms. If you dive in water it's different. The relative weight, that is load, of your body in water seems lighter because of the water's greater density, and it's dense enough that you can float and dense enough you can flap your arms and swim. You can push against it and propel yourself forward. Well, water presents a better impedance for the work that needs to be done and the load is a better match too.

When you put together a system you want to make certain that you are not asking your equipment to do a job it cannot do due to this kind of mismatch. This hold true for the amp speaker connection too! Even more so because a speaker presents a variable load to the amp.

Sincerely, I remain
A preamp is also an amplifier. It amplifies the signal from the source to the amplifier. Percentage wise it amplifies the signal more then your main amp. It is probably the most underated piece of equipment in high end audio.