Apple Training Series: Mac OS X Support Essentials v10.6: A Guide to Supporting and Troubleshooting Mac OS X v10.6 Snow Leopard

The kernel initializes the Input/Output Kit (I/O Kit), which controls input and output devices. The I/O Kit links the loaded drivers into the kernel based upon the device tree previously created by Open Firmware.

The kernel starts the launchd process, and the launchd process is then responsible for bootstrapping the rest of the system.