QMK (Quantum Mechanical Keyboard) is an open source community that maintains QMK Firmware, QMK Toolbox, qmk.fm, and these docs. QMK Firmware is a keyboard firmware based on the tmk_keyboard with some useful features for Atmel AVR controllers, and more specifically, the OLKB product line, the ErgoDox EZ keyboard, and the Clueboard product line. It has also been ported to ARM chips using ChibiOS. You can use it to power your own hand-wired or custom keyboard PCB.
If you plan on contributing a keymap, keyboard, or features to QMK, the easiest thing to do is fork the repo through Github, and clone your repo locally to make your changes, push them, then open a Pull Request from your fork.
Otherwise, you can clone it directly with git clone https://github.com/qmk/qmk_firmware. Do not download the zip or tar files; a git repository is required to download the submodules in order to compile.
Before you are able to compile, you'll need to install an environment for AVR or/and ARM development. Once that is complete, you'll use the make command to build a keyboard and keymap with the following notation:
make planck/rev4:default
This would build the rev4 revision of the planck with the default keymap. Not all keyboards have revisions (also called subprojects or folders), in which case, it can be omitted:
make preonic:default
QMK has lots of features to explore, and a good deal of reference documentation to dig through. Most features are taken advantage of by modifying your keymap, and changing the keycodes.