qmk_firmware/docs/Differences-from-TMK.md

Understanding the essential changes made on the tmk_keyboard firmware should help you understand the QMK Firmware.

Firmware	TMK	QMK
Maintainer	hasu (@tmk)	Jack Humbert et al.
Build path customization	TMK_DIR = ...	include .../Makefile
keymaps array data	3D array of uint8_t holding keycode	3D array of uint16_t holding keycode
fn_actions array data	1D array of uint16_t holding action code	1D array of uint16_t holding action code