Vocabulary question

Some amplifiers will not have the power reserves to reproduce a sudden burst of musical energy. An amplifier which does possess this reserve power is able to reproduce this difficult passage of music. When the sudden burst of musical energy occurs the amplifier reproduces it immediately and completely. A lesser amplifier will trim detail and not provide the sudden impact of this piece of music. This is where the expression 'fast' is derived.

I hope this makes sense to you. You might also run down to Barnes and Noble and grab a copy of Robert Harley's "Complete Guide to Highend Audio." His book has many of the definitions a person might be seeking.

For me an amp which is fast is one in which the leading edge of a note (burst of sound) rises from nothing to full reproduction in a very short time. Think of a graph with a flat line which, when there is a burst of energy, can move perpendicular to its original path, i.e. 90 degrees (which would be as fast as it can get) or could rise on an lesser angle which would make a duller sound. (The correlation to rise time is how long it takes an amp(and speaker)to release a sound, i.e. damping). An amp which is fast can greatly enhance the sense of pace and rhythm of music if your speakers are up to it.

The most colloquial description I can think of is that sudden dynamics in the music seem to leap out of the speakers at you. For example rimshots or a hard snare drum hit, or the pluck of an acoustic guitar. It's hard to nod off when listening to such an amp.
In contrast a "slow" amplifier might have an equally detailed sound, but simply puts you to sleep. This might be a good thing if you like to relax to music.
It's all a matter of taste (though my personal taste is that "fast" amps sound closer to live music). If an amp gets too "fast" it can be very tiring to listen to for extended periods, though it could sell very well because of the wow factor during short demos.

Thanks guys for all your response....This really give me a better feel for a "fast" amp......With this in mind, does a tube amplifier sound slow?.....(e.g. Hovland Sappire only rated at 40 watt per channle)...

Both tube amps and SS can be slow or fast sounding - a lot has to do with how they integrate with your speakers. I think speakers are a bigger problem than amps - one reason that a lot of folks like panels and electrostats - they are faster and do not retain energy as long as most dynamic speakers resulting in a clean and crisp sound. I'm not familar with the Hovland and I don't think you mentioned your speaker.