Overview of a computer boot sequence
When a computer boots, it first runs firmware stored in persistent memory. On PCs, this firmware is called the BIOS. If you have a problem at this stage, it's off-topic for this site, since Unix is not involved yet, but try asking on Super User.
The firmware then loads a bootloader, typically from disk or from the network. Although bootloaders are not part of the operating system proper, questions about bootloaders typically associated with unix and Linux are welcome on this site.
The bootloader loads the operating system kernel. The kernel initializes itself and some hardware devices, then on typical Unix systems runs the
init program. Init in turn starts system services, including programs that present a login prompt.
- grub (and grub2): a versatile bootloader used by many Linux distributions
- lilo: the traditional bootloader for Linux on PCs
Kernel boot sequence
- initrd, initramfs: on Linux, a virtual RAM disk that is loaded by the kernel before the “real” OS starts. The code in the RAM disk typically loads additional drivers (modules).
Unix boot sequence
- init: process number 1
- init-script: scripts that start and stop services, invoked by init at boot time
- upstart: a replacement for the traditional init program