With Apple's iTunes Music Store (iTMS) at last available outside of the US, it has become easier to track down old favourites and to discover new musical styles. But once your music library is of a certain size it is sensible to organize it using iTunes playlists; this organization will often involve classification by genre. For the non-musicians among us this can pose something of a problem. Of course we all recognize rock music when we hear it, right? I mean, you couldn't confuse it with folk or pop or blues could you? Likewise, hip-hop, rap, and reggae are pretty distinctive styles aren't they? The more you get into it, the less certain you become...
Let's see whether Wikipedia, which I find to be an increasingly important reference site, can help. But first—an obvious question—why classify at all?
Classify this, baby...
Some of us like to be organized. We enjoy seeing order from chaos; method from madness; compartmentalizing; characterizing; describing. Everything has it's place. How much clutter is on your desk?
Biologists use a taxonomy to classifying species; librarians employ a library classification system. In both cases these systems help us relate items in our mind on the basis of shared characteristics. Perhaps hierarchical classification is a natural tool of the human mind, integral to cultural and social systems? Humans organize themselves on the basis of shared goals; it seems only logical that we should organize the world around us on the same basis.
Does music have a goal?
Music certainly has a theme, and that theme is described by the primary melody. You could argue that the melody is directed at achieving a "goal state" in the listener—be that clam, excitement, appreciation, the desire to dance, or whatever. But the melody per see cannot form the basis of a classification system; different melodies can achieve the same goal state, and any given goal state can be achieved by different melodies. So there must be some other basis to the classification of music.
Genre
A musical genre is a category of music sharing a common style or elements (including historical age). As Wikipedia points out, there is controversy over how to categorize various styles, and debate as to whether this should be attempted at all. There is a huge list of possible genres, which to some extent counteracts the common desire to pick a genre to match your mood. And so we come to the practical point: academic arguments aside, most people probably want to use genre to find "more similar to this" or to listen to music that reflects a very general mood.
Your mood: your choice
As no one agrees on the classification system, you can pick-and-choose what genre you want to "recognize", or alternatively invent your own! That said, Wikipedia does identify a number of "widely agreed" major genres. Having increased my awareness of the difficulty of classification, and of the incestuous relationship between many musical styles, I now find myself less concerned with the "accuracy" of my own style-based playlists.
I am a fan of oldies music and a computer programmer too. I tried to classify my MP3 collection, but none the available tagging programs fits my needs. Therefore, I write my own program, which uses more option from the ID3V2 standard than the "holly seven" (Title, Artist, Album, Track Number, Genre, Year and Comments).
Now I want to add a classification of "Mood", but I can't find a list of known moods.
Does someone Know of such a list?
Gideon, I see lots of results from a Google search using the terms "music mood classification". I presume you tried this already; is none of it helpful because you are after something very specific?