An amplifier, or power-amp, supplies the power that drives your loudspeakers.
A pre-amp, or processor, does all the decoding of the incoming audio stream from the DVD/cable/sat/etc., adjusts volume, selects source, etc. AND sends it to the power-amp. Most newer pre-amps have built in equalizers and set-up programs that automatically adjust the level (volume) of each speaker, sets distances, etc.through a supplied microphone that you place in your seating position.
An A/V receiver generally combines eveything into one piece of equipment.
Pre-amps and power-amps are generally considered PREMIUM or HIGH-END and as such, they usually cost much more than a simple receiver. They're also built to a higher standard.
Some of the advantages are that with the amps seperated from the processing, there's no electrical interference or "crosstalk" with the processor caused by the amp section.
Another is that a power amp will generally have MUCH more power than a receiver. More power equals more "headroom" or reserves for musical or cinematic peaks while listening to music and watching movies.
So yes, they are superior, but you really need at least some decent speakers to exploit the benefits.
Some people pair a power amp with a receiver, using the receiver as a pre-amp to reap the benefits of the REAL power of a seperate amp. Most $500-800 receivers rated at 120watts by 7 channels only put out 60-70 watts when you're actually using all 7 speakers.
Here is an used Rotal power amp that would blow away any sub $1,000 receiver in clean, reserve filled power output :