Dimensions of style
Tagging digital music by genre is common, almost required. Genre helps you navigate and search your music collection, and helps build playlists. Assuming your genre tags are both completed and in a usable state, most digital music players allow you to browse your music by genre.
Genre tags have limits, though. You might choose genres from a pre-defined list, but what if the genre you want to browse by isn't in the list? What if you want to group music by genres that are not necessarily defined 'genres' but describe the music in different terms; the style of the music, its mood, or the time at which it is intended to be played? All these dimensions could be described as genre in one way or another, but digital music offers alternative, more specific tags by which you can better describe your music.
The 'mood' of music reflects the emotions or feeling music elicits, whether romantic, sad, joyous or melancholy. AllMusic have an exhaustive list with examples of songs, albums or artists that exhibit each mood.
While many genres are synonymous with certain 'moods' others tend to allow for a variation. Emo or Big Beat offer generally fairly constrained opportunities for different emotional communication, while Rock music in general or Classical music conveys all sorts of moods. It seems the more generically you identify your genres, the more scope there is for variation in mood. So, if you have decided to employ a coarse grained genre classification, mood might be a useful way of subdividing your collection. After all, while both The Beach Boys and The Smiths could be described as 'Rock' music, their tones and moods could hardly be more different.
To tag music with mood information, you can normally provide a generic tag with the field name 'MOOD' and the textual data being the mood you want to use. For MP3s with an ID3v2.4 tag, you can use the TMOO field.
Just as with genre, it's best to decide on a list of acceptable moods and try to pick from there to avoid your mood classifications becoming unwieldy. The AllMusic guide might be a good place to start.
Music, being an art form, is often used to communicate themes and concepts. Some might be religious themes, some political, some themes of love.
There is generally no accepted or defined way of tagging theme. Create a user defined tag and call it THEME. To my mind, the values THEME might take could really be extremely expansive and it's difficult to imagine constraining by a pre-defined list.
Slow, fast, very fast, speed metal?
Before I cleaned them up with bliss, a number of my albums used to have a genre tag of 'christmas'. 'Christmas' isn't really a genre as such, but it is a useful way of classifying music.
This is because hosting parties and visitors is common at this time, so being able to classify your music by occasion can make it easy to build playlists for the visit. Occasions can apply to different religious holidays or also personal events, such as children's birthdays.
Occasion is less supported by music players. If you have a music player that supports user generated tags I'd suggest using the field name 'OCCASION' and choosing from events defined in Wikipedia such as religious festivals or social events.
How music players work with these tags
Mood, occasion and other similarly infrequently used tags are often not supported by music players. To be able to browse or search by these tags you need a music player capable of searching by arbitrary tags. MediaMonkey is an example of this, as is foobar2000.
Even if your favourite music player cannot browse or search by these classifications, you can work around it by using one of the players above to generate a playlist. Export the playlist to a file and you can then open it in your favoured music player.
So many dimensions, so many re-categorisation opportunities...