Talking about high-end audio systems is like talking about wine: There's a language one has to learn, one that really has to be acquired by experience. But nonetheless, there is a bit that can be said about what some of the common terms mean, which can be helpful to the neophyte.
One can divide aspects of reproduction into three basic categories: Musical, Tonal, and Spatial. The Musical aspects concern such things as how faithfully the timbre of instruments is reproduced. The Tonal apsects concern such things as the depth and 'tightness' of bass reproduction, or the extension and clarity of the treble, and the naturalness of the midrange. And, finally, the Spatial aspects...these are harder to explain: I will spend a good deal of time on that below.
Musical Aspects of Reproduction
There is little 'lingo' that goes with the musical aspects. But it is worth saying a bit about them, as they are among the most important. Anyone who has ever heard an oboe and an English horn knows, first, that these instruments sound very much alike and, secondly, that they sound different. A poor system will make them sound the same: One will be completely stymied if asked to say, with any confidence, which of the two someone is playing on a given recording. But a good system will reproduce the instruments faithfully; it will make an oboe sound like an oboe and an English horn sound like an English horn. There's an obvious, musical benefit to that sort of accuracy. But it goes deeper. Violins, or guitars, do not all sound the same. And, even where one has a single violin or guitar, it will sound different depending upon who is playing it. A poor system will mask these differences, too: A good one will reveal them. And that will give the listener a much better sense for the individuality of the performance.
I might also include here what some reviewers describe as 'rhythm' or 'pacing'. It is very hard to say exactly what these terms mean and much easier to learn to distinguish them by listening. But, roughly, what is meant is: Does the system properly capture the rhythmic sense of the music? does it swing, in a relaxed sort of fashion, when it's meant to swing? and drive fiercely forward when it's meant to do that? For myself, this is a very important feature: I sense that a system is doing poorly in this respect when I put on the Allman Brothers and they sound like they're half asleep; their music is meant to rock, to get your feet tapping.
Tonal Aspects of Reproduction
There is more of a lingo to describe tonal aspects of reproduction. One speaks of a system's bass as being 'tight' or 'muddy'; of midrange as being 'lean' or 'bright'; of the treble as being 'harsh' or 'rolled off'. Most of these terms are largely self-explanatory: To know, however, when a system has really tight bass, one needs some experience; a system might sound to you like it has tight bass, but that's because you've never heard a system that has really tight bass.
There is a great deal of difference, between individuals, regarding what sorts of omissions and commissions they can tolerate here. In the bass, tightness has to do with how much control the system has over transient information: When the string of a double bass is plucked, does the note start quickly? or is it smeared, so to speak? Can one really pick out the individual notes that are being played? or does the bass become an almost monotonal mess? To my ear, though not to everyone's, clean, tight bass reproduction, with good distinction of pitches, is much more important than really deep bass reproduction. My current reference system starts falling off about 40Hz: There's information that's missing, and one can hear it on certain other systems; but I don't really miss it all that much, musically speaking. I guess that's partly becuase I'm not that into pipe organ. If you are, you'll want deep bass.
Harshness in the treble is annoying to almost everyone. (Really harsh treble actually gives me a headache.) It's easy to hear this sort of thing on cymbals: A poor system makes all cymbals sound alike, like trash-can lids; a good one gives you a real sense for the differences between, say, ride and crash cymbals and reproduces their bell-like qualities faithfully. A rolled-off treble is too low in volume, tending to make the whole presentation sound a little flat and uninvolving. But, it should be said, some treble roll-off can sometimes be a life-saver in a system other of whose components have a tendency towards harshness.
The language used to describe midrange reproduction is probably the most difficult to master. To be honest, I can't say I've gotten it myself. But, fortunately, it is easy to evaluate this feature of a system's performance: Just toss on some good female vocals. My own favorite tests are k.d. lang's "Outside Myself", from Ingenue, and Toyah's voice on just about any cut from Sunday All Over the World's Kneeling at the Shrine. k.d. should sound lush, smooth, breathy: A good system reveals these qualities; a poor one can make her voice sound thin, if it fails in one way, or a bit raspy, if it fails in another. Toyah has a voice with some harsh qualitites, and she tends to accentuate certain sibillants ('s' sounds, for example): A poor system will make her sound too smooth or, instead, too Dylan-esque and almost unlistenable, emphasizing the sibillants in a way that makes the music sound far too 'reproduced'.
All aspects of tonal reproduction are important, but it is usually said that the midrange is the most important. (It is because tube gear tends to do such an incredible job with the midrange that so many people fall in love with it.) It is here that our ears are most sensitive, and it is here that most of the music usually takes place. One can put up with something lacking in the bass; one can put up with a slightly rolled-off treble. One isn't always listening to the bass or treble, as such. But it is hard to put up with a harsh midrange: That tends to make everything sound harsh.
Spatial Aspects of Reproduction
Most of the lingo one encounters is used to describe the spatial aspects of reproduction. What is meant here is roughly this: An orchestra, say, plays in a hall; the players are in different places and the hall itself has sonic properites. So how accurately are these portrayed? We can divide the spatial aspects themselves into three parts: Imaging, Reflection, and Soundstaging.
Imaging has to do with the presentation of individual instruments and voices. Are they where they are supposed to be? How precisely are they located? Does their apparent size correspond to their actual size? Do the instruments blur into a mass? Or is the space between them clearly empty? Most of the terms one hears are used to answer one or another of these questions and are, in that respect, self-explanatory.
Imaging is important, muscially, because it allows one to distinguish individual instruments and therefore hear the individual lines that are being played. It also contributes, for me, to a sense of the 'reality' of the presentation, which is just plain fun. That is also true of the other aspects of spatial reproduction.
Reflection has to do with the relationship between the performers and the space in which they are performing. How clearly is the sound that emanates from the instrument itself distinguished from what is reflected by the room or hall? Does one have a sense for the hall's back wall and for the reflections coming from it? for the side wall?
Soundstaging has to do with the overall presentation of the space. How wide is the apparent space in which the players are arranged? How deep is it? Do the timpanis in the back seem to be way in the back? or have the percussionists rudely set their drums on the violists' heads (in which case the soundstage is 'compressed')? And where do you, the listener, seem to be? Do you always sound like you're front and center (in which case a system is 'forward')? Or do you feel like you're out in the lobby (it's 'laid back')? Does your apparent position change, as it should, with different recordings?
This is important, musically, because it affects how well a system can image: If the soundstage is narrow and shallow, there simply isn't much 'room' for the instruments to occupy: An orchestra can not but sound bunched and indistinct.
Things get complicated, in part, because the answers to questions about the spatial aspect need not be the same throughout the frequency spectrum. One will, thus, sometimes hear it said that a particular component's treble is forward. What this means is that the treble seems to come from more forward in the soundstage than other bits do. That can be most annoying, as it destroys the coherence of the spatial presentation. Perhaps that is the most important thing, that the spatial presentation should be coherent: Even if the soundstage is not terribly deep or wide, what one would most like is a presentation of what could be, even if it was not, the actual space in which the performace took place.
Of course, you will only hear these sorts of details on "natural" recordings made of real instruments in real space. Many great jazz recordings from the 1950s and 1960s were like this, though certainly not all. The classical recordings from the so-called Golden Age of Stereo—the RCA Living Stereo recordings, for example, from the 1950s and 1960s—are still a reference for many people. A great test is Miles Davis's classic Kind of Blue, which was recorded direct to three-track and has a very natural presentation. But even with multi-tracked recordings, many of the spatial factors come into play, though perhaps in unexpected (or even unwanted) ways. On many of the songs on Joni Mitchell's great album Hejira, for example, one can quite clearly hear that she is singing in some tiny box.