Here are the classes, structs, unions and interfaces with brief descriptions:

CAnimClip	An animation clip — ROM-resident, 12 bytes
CAnimPlayer	A playback head — RAM, 8 bytes, array-friendly
CAudioSample	BRR sample descriptor
CAudioVoiceState	Voice state information
CBgAsset	A full background bundle: tileset + tilemap + map size
CEffectState	Tracks which HDMA effect is currently active and its parameters
CEntity
CGameState	Centralized game state structure
CGfxAsset	A tileset bundle: graphics + palette + color depth
CLineClearResult
CMetaspriteItem	Metasprite item structure (PVSnesLib compatible)
CMonster	Sprite state structure holding position, animation, and direction
CNpc	A villager: where it stands and what it says. The struct shape and the rows both come from the Entities layer of town.tmj, so adding one is a map edit
COamDynamicConfig	Configuration for the dynamic sprite engine
CPanel	A 9-slice panel layer: one BG tilemap and the sheet to stamp from
CPlayer	Per-player position state
CRect	Axis-aligned bounding box (rectangle)
CSaveState	Data structure representing one save slot's persistent state
CScene	A single scene's callbacks
Ct_objs	Game object data structure
Ct_sprites	Dynamic sprite state structure (16 bytes, PVSnesLib compatible)
CTextConfig	Text rendering configuration