Here's a quick no nonsense guide to how linux boots up.
- When a linux system is started, Linux Loader (GRUB or LILO usually) loads the kernel.
- Initially, when LILO or GRUB calls kernel, the kernel is uncompressed and kernel starts checking hardware attached, loads the required modules.
- When all hardware modules or drivers are appropriately loaded, Kernel opens root volume (/) in read only mode.
- Kernel checks if root volume has any problems. If no problems are detected, the root volume is mounted as read-write.
- Once the above step is done, kernel executes a process called init.
- When init is done, the linux is up and running.
initis highly configurable with various customizations.