The boot order is the following(semplified for clarity):
1)First the BIOS (or coreboot combined with seabios) get executed. The BIOS or coreboot+sea bios reside in a flash memory that is on the mainboard. 2)Then the BIOS(or coreboot) loads grub grub usually resides on a hard disk. 3)grub which loads a kernel The kernel usually resides in a filesystem on a partition under /boot/ 4) the kernel once it finished booting usually loads /sbin/init and then the boot process continue in userspace(the init daemon loads udev, and various services, and at the end your display manager which at the end will start your desktop environment). /sbin/init usually resides in a filesystem on a partition under /sbin/init
I think wikipedia should have many pointers on the boot process.
Denis.