I've heard the process described in more detail than this, so what I've got is a work in progress. I'm going the GRUB way if that matters.
1. Power On Self Test (POST) - Computer's Power button is pressed.
2. If the BIOS and POST recieve no errors, the BIOS loads and executes the first 512 byte from the hard drive.
3. The bootloader is executed which brings up the kernel selection menu
4. Grub loads stage1, and 1.5 (1.5 loads the filesystem that /boot resides on)in order to bring up the menu.
5. Once the kernel is selected and loaded into memory, it initialized devices and loads the ramdisk. It mounts / in memory temporarily in order to mount the virtual filesystems before performing the 'pivot_root()' function to allow / to be mounted on the hard drive.
6. if the ramdisk exists, it will load any modules dependancies and execute the pivot_root
7. INIT - init gets executed. Init reads the /etc/inittab which will determine the runlevel.
8. Runlevel. Since the devices and runlevel are in place, it now starts spawning processes via the init scripts stored in /etc/rc#.d/