How does the Ubuntu boot process work?

I have read many articles about the Linux boot process. As I understand the Linux boot process is something like this :

BIOS (UEFI) → MBR → grub → Kernel → init  

What I really don't understand what the etc/sbin/init starts and what the relation between the Ubuntu upstart daemon, /etc/init.d and run levels is...

Solution 1:

2 resources that will help you understand the boot process better:

1. bootchart

   sudo apt-get install bootchart
   sudo reboot
   

   Now every time a chart like this one will be created in /var/log/bootchart/ every time you reboot. You can learn a lot by looking at a boot chart of a system you know intimately: yours! ;-)

2. IBM's manual Inside the Linux Boot Process