NewOrder started an interesting concept in the "A Look at Metal Music" topic: Can, or Should, a song be classified into a specific genre or subgenre of music? I suspect strong opinions on both sides of this discussion. Please try to keep flaming and harsh words out of this discussion. Thanks. B)
I would like to say Yes, but categorizing music is not as simple as it appears. The simple, broad genres such as Rock, Techno, Country, Classical (technically "Orchestral" is a better word, since "Classical" refers to an era in musical history, not the orchestral style of music), etc.
"Rock", "Classical" and "Techno" (I prefer the term "electronica" since techno can be considered a subgenre) are probably the most overused and least understood terms when describing music. These three genres are actually "Major Genres", if you will, with the songs fitting into "subgenres" of each of these. There are, of course, other "Major Genres", such as Country (iffy, since it can relate closely to rock), Ethnic (from a foreign culture), Bigband, Choral, A-capalla (related to choral), etc. There are probably others which I can't recall at the moment.
These definitions of main genres are purely off the top of my head. They are certainly up for debate! Feel free to add as you wish. 
A major genre is basically defines the instruments and general style with which the music is played. I tend to think of the following:
Rock: Consists of a small band (up to about 6 members). Features at least 2 guitars (lead and bass) and a drum kit. Other additions are at the discretion of the band, which help to decide the subgenre.
Orchestral: Features either a small (small variety and quantity of orchestral instruments) or full-sized orchestra. The amount of focus placed on the orchestra (whether the receive all of the attention or if they are alongside a rock band or other insturments) helps to determine the subgenre.
Electronica: The music was generated by synthetic means, by use of
computers,
software or hardware synthesizers,
or audio samples -->(this is what I do
).
The quantity of live performance and the style of the music helps determine the subgenre. Hip-hop and rap generally falls in as a subgenre of electronica, as a huge percentange of hip-hop music is synthetically-generated.
Country: The modern version of this genre can be considered very similar to rock, with the general differences being the focus on pianos, fiddles, or other "honky-tonk"-style instruments, the lyrics of the song and the singing style. Classic country can be considered significantly different from rock and "modern" country.
Bigband: Refers to a typical jazz band: trumpets, trombones, saxophones, possibly flutes and clarinets. Similar in a way to orchestral, but a more clearly defined genre.
Choral: The primary focus of choral music is a group of human voices. They are often used in conjunction with an orchestra as well. A choir is generally a large group (perhaps 8 or more singers, divided up into approximately 4 (typical) parts) Pure Choral music is not common in popular culture.
A-capella: Similar to choral music in that it relies on a group of human voices to produce all of the music. Generally, A-capella is a smaller group than a choir (maybe 4 or 5 singers. 4 is typical).
Spoken Word: Usually performed by a small group (1 or 2) with electronic or acoustic background accompaniment. Rap and spoken-word versions of songs fall into this category.
Now comes the fun: Nearly all songs do Not fit nicely into a single genre. A song that drifts into two or more genres, I tend to refer to as a "fusion". For example, one could say this song is a "Rock/orchestral fusion" if Rock is the primary (most prominent) element of music in the song, or an "Orchestral/Rock fusion" if orchestral is the primary element. Of course, a song could easily spread 3 or more genres. Electronica is an incredibly common genre to appear in music today, because it is so easy for anyone to add a drum beat or a few background effects to their song. Name off 10 pieces of music and probably at least 5 of them have electronica elements to them unless they're pure orchestral. Most "dance" songs today (with some exceptions of course, such as jazz) will consist of some element of electronica (usually the drum parts are computer-generated).
SubgenresThis is where categorizing music gets really interesting and complicated. While a song may be Rock, is it more like Song A or more like Song B? This is one basis behind defining the subgenre of a song. Of course, sub-subgenres exist but I'm not going there just yet.
Rock, for example, can be broken down into these (and many more) subgenres:
Ballad Rock / Soft Rock
Classic Rock
Hard Rock
Metal (which is argueably a different Major Genre but I like to put it within rock because of the strong musical similarities)
Punk Rock
Emo (If you really want to go there
)
Math Rock (don't ask...it does exist)
All of these subgenres can be further broken down into sub-subgenres to help group similar songs together and to help describe the sound of a particular song.
Tails and I had a discussion on MSN about whether the "Pop" genre exists. While a conclusion wasn't reached, I hoped to address it here. I am thinking of songs by bands like
ATC (A Touch of Class), Aqua and the like. I personally am starting to agree with Tails that "Pop" does Not really exist. Instead, it's a pseudo-genre, based on the song. In the case of "Around the World" by ATC, the song could be categorized as:
Electronica -> Techno -> Upbeat -> Dance -> Vocal -> 21st century -> Year 2000
...and so forth. I would eventually define this song as being "21st-century vocal techno/dance", to be reasonably accurate. A more precise definition could be made is desired.
One can break down songs into smaller and smaller groups until there is only a single song in the group. It is similar to how species are scientifically categorized.
A more complex genre breakdown would be for a song like "If We Hold On Together" (the Diana Ross version). Let's try:
Rock -> Soft rock -> Other lyrical topic (not love song) -> Inspirational -> Feat. Piano -> 1980s -> 1988 -> ...
Orchestral -> Background orchestral elements -> ...
Electronica -> Downbeat -> Ballad -> Background -> Backgound synthesized sounds -> ...
That being said, "If We Hold On Together" could be classified as
1988 Inspirational Soft rock with orchestral and synth background. One could go further if they wanted to.
