The wayland protocol is essentially only about input handling and buffer management. The compositor receives input events and forwards them to the relevant client. The clients creates buffers and renders into them and notifies the compositor when it needs to redraw. The protocol also handles drag and drop, selections, window management and other interactions that must go through the compositor. However, the protocol does not handle rendering, which is one of the features that makes wayland so simple. All clients are expected to handle rendering themselves, typically through cairo or OpenGL.
The weston compositor is a reference implementation of a wayland compositor and the weston repository also includes a few example clients.