It sounds more like he's asking how the boot process works. Basically, the computer's BIOS loads the bootloader into memory. The bootloader parses its configuration file and (possibly) waits for user input. After some timeout, it boots the default OS. It reads the kernel off of disk (Linux kernels are compressed, so the kernel image must be uncompressed too), loads it into memory at some predetermined address, and executes a jump instruction to that address. The kernel then ges through hardware detection, initializes itself, and spawns the init process (pid 1, the first user process to run). Assuming we're using a standard init, init parses /etc/inittab to figure out what it's supposed to run. Eventually, depending on how it's configured init will launch the getty process on the console, presenting the user with the standard login prompt. It may also launch xdm, kdm, or something similar to give the user a GUI login prompt.
That's the process in a nutshell. Bt198629, if you want more detail than that, you'll need to ask a more specific question.