This is an archive of past FreeBSD releases; it's part of the FreeBSD Documentation Archive.
FreeBSD uses a three-stage bootstrap by default, which basically entails three programs which call each other in order (the two boot blocks, and the loader). Each of these three build on the previous program's understanding and provide increasing amounts of sophistication.
The kernel is then started, at which time devices are probed for and initialized for use. Once the kernel boot process is finished, the kernel passes control to the user process init(8), which then makes sure the disks are in a usable state. init(8) then starts the user-level resource configuration which then mounts filesystems, sets up network cards to act on the network, and generally starts all the processes that usually are run on a FreeBSD system at startup.