Most of the Galleries are pretty self-explanatory... the character-specific folders get fanart of said character, with the "Other Music Ponies" folder covering the remainder that aren't quite popular enough (yet) to receive his or her own section. The 'OCs' gallery is reserved for music-themed Original Characters, and the Cosplay and Fanfiction galleries require no explanation.
The other galleries are for the following:
Ensembles: Fanart containing 2 or more music ponies, or any other group picture of ponies that has something to do with music, and contains at least one "official" music pony.
Amateurs: Any non-music canon character(s) engaging in a musical activity of some kind.
Pony Album Covers: Album artwork featuring MLP characters; both original albums and parodies on real-life albums.