I am a lifelong composer and pianist, and have been a double bassist with a major American symphony orchestra for over thirty-five years. I have reintroduced this topic as it seemed to degenerate into something that did not do it justice. This is an important topic of which I wish more musicians had a better grasp.
Most of the previous discussion centered on Classical music as a style. But Classical music is no more a style than improvisation. Classical music is a method of creating, communicating, and realizing music. It is most significantly characterized by the fact that the communication is done VISUALLY. A style of music is determined by the way in which the music sounds. As far as I can determine, there are essentially three methods of creating/learning music: to read or write it, to improvise it, and to learn and/or perform the music by ear. Any style of music can be learned, created, or performed by any of these three methods. A Classical musician is someone who makes music by reading it.
That said, there are several characteristics of Classical music which have been brought about by the fact that it IS visual. A composer must know what is going to happen when he or she writes a particular note for a particular instrument with a particular dynamic, length, articulation, etc. A performer must know what those representations mean. Both participants must have an abstract understanding of what is expected. It is this abstraction that makes it "classical". Classical performers (and composers) strive toward certain classical "ideals" of perfection in tone, execution, rhythm, beauty, and, yes, style. Popular or jazz performers, by contrast, strive for individualism in most aspects of their performance. Only a real connoisseur can tell the difference between classical performers, whether individual instrumentalists or ensembles. This is because most classical performers are striving for similar ideals of tone, etc. However, many average listeners have no trouble distinguishing between popular performers and bands. For a popular performer, the more distinctive the better!
Because the music is represented visually, it is subject to the same scrutiny as any other visually represented artwork. Time is represented spatially, and composers (and performers) can grasp, can actually SEE an entire work at once. It can be edited, rewritten, and perfected. Composers have drawn from other visual representations, such as literature and poetry to craft their work in similar ways. Complexly integrated musical form is the direct result of this visual representation.
But these characteristics can be brought to improvisation and aural music as well. When I was growing up, Classical was the "serious" music with other types of music being intellectually and (especially) socially inferior. But what started as "hi-fidelity" recording changed all that. Not only did the great Classical music reach the masses, but ALL music reached the masses, and it reached them without social stereotype. As Marshall McLuhan stated so insightfully in the 1950's, hi-fidelity recording made all music socially equal, because it all emanated from the same two speakers. Very shortly creative stylistic hybrids of all sorts started to emerge, in both Classical and popular music. Slowly the tightly controlled markets began to break-up, and the days of linear stylistic "progress" were gone forever. The internet has continued the job that renegade labels began in the 1950's and 60's. Since the internet took over, it is the availability of music from all over the world that has particularly impressed me.
But there are hybrids in performance as well. Using the big bands of the 1930's and 40's as a model, orchestras have been performing concerts with a soloist or group from another kind of music for 30+ years. With the orchestra, I have performed concerts with Pop and Jazz performers, Rock'n'Roll, Country, Western, Bluegrass, Folk, Celtic bands, African drum ensembles, Gypsy bands, Native American drums, dancers, and vocalists, Native American flutists, a Didgeridoo soloist, Japanese and Chinese instrumentalists and ensembles, and many more. In all cases, the orchestra was playing the STYLE of the artist(s) we accompanied, but we were playing in a Classical manner. It is a hybrid presentation. A recording studio often works in the same way, hiring Classical musicians to "back" a popular artist and writing out their parts.
With multi-track recording and computer technology, the editing, reworking, and spacial organization, once the sole custody of Classical music, have been brought full-force into non-written music. The proliferation of recorded music has also diluted the skill level of Classical musicians. I remember when the Suzuki method of teaching violin came out in the 1960's, it was highly controversial. Learning Classical music by ear was not only blasphemous, it was cheating! Suzuki was just using methods that had been common for millennia in Asia, namely "teacher plays - student imitates." But Suzuki used not only the teacher but recordings of great violinists! Now this has become the norm, and everybody listens to the same performers. I just played a concert in which two local fourteen year-old soloists played movements of the Dvorak Cello Concerto and the Rachmaninov Second Piano Concerto. Their performances were musical, sensitive, expressive, and technically competent. This would have been unheard of forty years ago. But the ear is much more sensitive than the eye, and its impression runs deeper. When I played rock'n'roll in high school in the 1960's, we routinely played four hour gigs from memory, and we were no prodigies.
The lines delineating Classical music become hazier each year. Most new music we play in the orchestra has been composed in MIDI and transcribed for orchestra. Is this really Classical? It is now! Composers used to take care in how they wrote for performers, as this was their only way to hear their creations. Now composers already know what the music sounds like. Now we get G melodic minor scales with A#'s and Gb's! The orchestra has changed too. For example, we have a trap drummer on contract, and our pianist has to read chord charts.
For most of my composition career, I have worked the in-between area of written music and improvisation. I have transcribed improvisations, used them as models, and used them as material. I have also tried to bring my composition techniques and aesthetics into my improvisation. In all cases, when the music is transcribed, it becomes something different. Try transcribing an improvisation, especially one that was done without reference to preconceived meter (or click track). You will begin to see just how fragile rhythmic notation is. If a musical passage is not CONCEIVED within the metric grid, it can be devilishly difficult to transcribe. What meter? Where is beat one? Meter change or syncopation? Written retard or metric modulation? Classical musicians think their music is very specific, but what makes it unique and dynamic is that it is NOT specific. It is abstract. A classical work is never played the same way twice. An improvisation or aurally conceived work is highly specific. Each sound is exact; each balance is perfect. A MIDI keyboard needs 127 steps to capture real dynamics. A sequencer divides a quarter note into 480 parts to capture real rhythm. An improvisation cannot be accurately transcribed because notation is not specific enough! And what a nightmare it would be if it was! It would make Pierre Boulez look like childsplay!
I am trying to be brief, believe it or not. This is a subject with a thousand side topics. A composer can only give certain clues as to how to perform his music. Imagine how DaVinci's Last Supper would be if DaVinci only wrote instructions as to how it was to be painted. But also imagine how many wonderful interpretations there would be! It is this constant reworking and diversity that makes Classical music the magnificent art form it is.