You might want to get Robert Harley's book "The Complete Guide to High End Audio", available at www.audioadvisor.com
Laid Back: The stereo image forms behind the speakers rather than exactly between them or in front of them
Forward: The soundstage forms in front of the speakers. Forward usually refers to the mid-range. Forward is opposite of Laid back.
Warmth: very subjective, but I use it to mean a rich, thick, syrpy sound-- notes in the mid-range are round and full. It can apply to all frequencies. The opposite(s) are cool, lean, thin. I prefer a slightly warm/rich music presentation to cool/lean, or analytical.
Black Back Ground: Actually refers to the "degree of silence" during the parts of the music that are supposed to be silent. Usually, the blacker (more silent) the better. A good black background also implies excellent clarity. A system that does not have a black background is less clear, more muddy, and has various electronic noise(s) associated with it, ie radio frequency interference (RFI) and electro-magnetic interference (EMI). It can be striking when you hear a system with a really black background. Cleaning up the electrical supply can promote blackness.
Aggressive: can refer to an excessively bright mid-range/treble, or an excessively forward presentation, or both.
Over the Top: Too bright, and similar to aggressive.
Bottom End Extension: Refers to how deep the bass of a stereo system goes. There are many adjectives to describe quality of bass.
Bloom: A desireable (usually) condition often best created by tube equipment whereby the soundstage is very full (holographic), and extends in all directions. However, sometimes stereo image coherence suffers with a strong "bloom". I like a good strong stereo image and am willing to forego bloom to get it. Bloom may also refer to thin and diffuse (as opposed to thick, tight and coherent).
Audiogon could use a reference to some kind of glossary for these kinds of terms-- there are many, many more. Cheers. Craig.