The bootloader is a small program (starts after the hardware check) that lets you choose when and how to launch the operating system.
The root filesystem is a relatively small filesystem, it contains all the important system files spread out in different directories under the root directory (top directory in the hierarchy of folders).
The kernel sits between user-programs and the hardware. When a program wants to access memory for example, the kernel takes care of that task.
The standard kernel is normally located at /boot/kerneland is started by the bootloader.
The kernel does the hardware check and mounts the root filesystem (/), it passes control to the script init.
Init is the first program that runs and has the process ID 1.
In FreeBSD we have two modes:
Single-user :mode for debugging purposes , in that mode the root filesystem is mounted read-only.
Multi-user : mode with the networking activated (default mode).
Startup options are controlled by the configuration file rc.conf.
Init is responsable for starting all the processes:
It checks if the clean flag is set on the hard drive, otherwise it runs fsck on it to correct the errors on it.
It mounts the filesytems as it is described in the file /etc/fstab.
It spawns the Shell and runs the scripts (programs written in Bash, Perl, Python, ...) in the directories /etc , /etc/defaults. Italso starts the scripts related to other servers located in /usr/local/etc/rc.d (scripts for web servers for example).
It starts getty or xdm (graphical version of getty) so we could log into the system.