audio systems basically have two stages of signal manipulation, a pre-amp, followed by a power-amp. These might be in separate boxes, or else combined into an integrated amp. If you add a tuner to an integrated amp, then you have a receiver. The above terminology dates from stereo and even mono days, but is also applicable to multi-channel A/V systems.
Basically the pre-amp stage does the signal manipulation/processing. In the old days this was basically just tone, balance, and volume controls, but might also include various filters, as well as phono input equalization and level adjusting (sort of a pre-pre-amp, raising the level of a phone cartiridge's couple of millivolts, to the 150 millivolt level of other analog "line-level" sources). Nowadays you also have decoding of digital source inputs, digital signal processing of sound fields, etc., and other functions that might be considered to be part of the pre-amp, but are sometimes viewed as separate signal processing functions. (In either case, they are before the power-amp.)
Pre-amps may provide a little voltage gain, but they have very little capability for driving speakers. Basically, they expect a relatively high-impedance load, and also typically have a pretty high internal source impedance (since it doesn't matter when you are driving a very high impedance load).
Power amps provide the oomph to actuall deliver power to your speakers. They present a nice high impedance back to the pre-amp (meaning they don't require much power from the pre-amp, nor do they load down the pre-amp). In turn, they have a nice low internal source impedance, so that they are capable of effectively delivering power to the low-impedance load presented by a speaker. A low source impedance also means that the frequency response of the amp will not be affected by the complex impedance versus frequency characteristic of the speaker. Power amps provide gain, and the capacity to deliver power to low impedances, but don't modify the signal in other ways (at least not intentionally).
Many receivers provide external access to the pre-amp outputs for each channel, just prior to the power-amp stage for each channel. Thus, if you want to use a larger external power-amp, you can use just the signal processing and pre-amp functionality of your receiver, and get the main amplification from the separate power amps. Since many multi-channel receivers are incapable of providing full rated power for all channels simultaneously, a common arrangement is to use separate power amps for the fronts, or maybe the fronts plus center channel, while continuing to use the receiver's internal amps for the surrounds, back-surrounds, etc.
A smaller number of receivers also provide access to their internal power-amp inputs, as well as the pre-amp outputs (you will usually see the two connected using little metal jumpers). This enables you to use a separate pre-amp/processor (or pre/processor) with your receivers power amps. But, since in most cases the "weak link" of a receiver is its power amps, rather than its signal processing, it is much more typical to use the receiver as the pre/processor, and separate power amps, than the other way around. (Also, the analog multi-channel inputs of the receiver allow you to use the decoders of your source, rather than the decoders built into the receiver, providing part of the flexibility provided by having power-amp inputs.)
Guitar players are very familiar with the concept of pre-amp and power amp, as guitar amps typically have separate gain/volume controls for both stages. If you want a clean sound, you keep the pre-amp volume/gain setting fairly low, and use higher gain in the power amp stage to get the loudness you want. Alternatively, if you want a distorted "fuzz" tone, you crank the pre-amp gain/volume way up, so that you are getting clipping of the signal being fed to the power amp. Then, you use a relatively lower gain/volume setting of the power amp (for the same perceived loudness as with the clean sound). Overall loudness is the same in both cases, but the intentional overloading of the pre-amp gets you a nice distorted rock guitar sound. (Or you can set both knobs to "11", a la Spinal Tap!)
(As an aside, tube guitar amps exhibit a fairly soft clipping, that generates a lot of second harmonics and a nice warm distortion. Transistor amps tend to clip "hard", which generates more 3rd-order harmonics, which tend to sound overly harsh. So, some transistor guitar amps actually use more complex signal processing techniques to artificially generate tube-like clipping, instead of real transistor-like clipping, in order to get a more vintage-sounding distortion.)
