At bootup time, the kernel spawns the process /etc/init. init is a program which reads its configuration file, /etc/inittab, and spawns other processes based on the contents of this file. One of the important processes started from inittab is the /etc/getty process started on each virtual console. The getty process grabs the VC for use, and starts a login process on the VC. This allows you to login on each VC; if /etc/inittab does not contain a getty process for a certain VC, you will not be able to login on that VC.
Another process executed from /etc/inittab is /etc/rc, the main system initialization file. This file is a simple shell script which executes any initialization commands needed at boot time, such as mounting the filesystems (see Section 4.8) and initializing swap space.
Your system may execute other initialization scripts as well, such as /etc/rc.local. /etc/rc.local usually contains initialization commands specific to your own system, such as setting the hostname (see the next section). rc.local may be started from /etc/rc or from /etc/inittab directly.