A computer without a program running is just an inert hunk of electronics. The first thing a computer has to do when it is turned on is start up a special program called an operating system. The operating system's job is to help other computer programs to work by handling the messy details of controlling the computer's hardware.
The process of bringing up the operating system is called booting (originally this was bootstrapping and alluded to the process of pulling yourself up "by your bootstraps"). Your computer knows how to boot because instructions for booting are built into one of its chips, the BIOS (or Basic Input/Output System) chip.
The BIOS chip tells it to look in a fixed place, usually on the lowest-numbered hard disk (the boot disk) for a special program called a boot loader (under Linux the boot loader is called Grub or LILO). The boot loader is pulled into memory and started. The boot loader's job is to start the real operating system.
The loader does this by looking for a kernel, loading it into memory, and starting it. When you boot Linux and see "LILO" on the screen followed by a bunch of dots, it is loading the kernel. (Each dot means it has loaded another disk block of kernel code.)
(You may wonder why the BIOS doesn't load the kernel directly — why the two-step process with the boot loader? Well, the BIOS isn't very smart. In fact it's very stupid, and Linux doesn't use it at all after boot time. It was originally written for primitive 8-bit PCs with tiny disks, and literally can't access enough of the disk to load the kernel directly. The boot loader step also lets you start one of several operating systems off different places on your disk, in the unlikely event that Unix isn't good enough for you.)
Once the kernel starts, it has to look around, find the rest of the hardware, and get ready to run programs. It does this by poking not at ordinary memory locations but rather at I/O ports — special bus addresses that are likely to have device controller cards listening at them for commands. The kernel doesn't poke at random; it has a lot of built-in knowledge about what it's likely to find where, and how controllers will respond if they're present. This process is called autoprobing.
Most of the messages you see at boot time are the kernel autoprobing your hardware through the I/O ports, figuring out what it has available to it and adapting itself to your machine. The Linux kernel is extremely good at this, better than most other Unixes and much better than DOS or Windows. In fact, many Linux old-timers think the cleverness of Linux's boot-time probes (which made it relatively easy to install) was a major reason it broke out of the pack of free-Unix experiments to attract a critical mass of users.
But getting the kernel fully loaded and running isn't the end of the boot process; it's just the first stage (sometimes called run level 1). After this first stage, the kernel hands control to a special process called ‘init’ which spawns several housekeeping processes.
The init process's first job is usually to check to make sure your disks are OK. Disk file systems are fragile things; if they've been damaged by a hardware failure or a sudden power outage, there are good reasons to take recovery steps before your Unix is all the way up. We'll go into some of this later on when we talk about how file systems can go wrong.
Init's next step is to start several daemons. A daemon is a program like a print spooler, a mail listener or a WWW server that lurks in the background, waiting for things to do. These special programs often have to coordinate several requests that could conflict. They are daemons because it's often easier to write one program that runs constantly and knows about all requests than it would be to try to make sure that a flock of copies (each processing one request and all running at the same time) don't step on each other. The particular collection of daemons your system starts may vary, but will almost always include a print spooler (a gatekeeper daemon for your printer).
The next step is to prepare for users. Init starts a copy of a program called getty to watch your console (and maybe more copies to watch dial-in serial ports). This program is what issues the login prompt to your console. Once all daemons and getty processes for each terminal are started, we're at run level 2. At this level, you can log in and run programs.
But we're not done yet. The next step is to start up various daemons that support networking and other services. Once that's done, we're at run level 3 and the system is fully ready for use.