This inheritance list is sorted roughly, but not completely, alphabetically:

[detail level 1234]

CAdafruit_BusIO_Register	The class which defines a device register (a location to read/write data from)
CAdafruit_BusIO_RegisterBits	The class which defines a slice of bits from within a device register (a location to read/write data from)
CAdafruit_GFX_Button	A simple drawn button UI element
CAdafruit_I2CDevice
CAdafruit_SPIDevice
CGFXfont	Data stored for FONT AS A WHOLE
CGFXglyph	Font data stored PER GLYPH
▼CPrint
▼CAdafruit_GFX
CAdafruit_GrayOLED	Class that stores state and functions for interacting with generic grayscale OLED displays
CAdafruit_SPITFT	Adafruit_SPITFT is an intermediary class between Adafruit_GFX and various hardware-specific subclasses for different displays. It handles certain operations that are common to a range of displays (address window, area fills, etc.). Originally these were all color TFT displays interfaced via SPI, but it's since expanded to include color OLEDs and parallel-interfaced TFTs. THE NAME HAS BEEN KEPT TO AVOID BREAKING A LOT OF SUBCLASSES AND EXAMPLE CODE. Many of the class member functions similarly live on with names that don't necessarily accurately describe what they're doing, again to avoid breaking a lot of other code. If in doubt, read the comments
▼CAdafruit_SSD1306	Class that stores state and functions for interacting with SSD1306 OLED displays
COLED_Display
CGFXcanvas1	A GFX 1-bit canvas context for graphics
CGFXcanvas16	A GFX 16-bit canvas context for graphics
CGFXcanvas8	A GFX 8-bit canvas context for graphics