The init program is a binary (ELF) executable file and it is the mother of all processes. It is always Process ID (PID) 1. Red Hat calls this the First Process.
The init program is really the end of the boot process and the beginning of the startup.
The init program runs the /etc/rc.d/rc.sysinit script and uses the /etc/inittab file to determine the startup configuration for Linux. It then runs the /etc/rc.d/rc script which checks /etc/inittab for the default runlevel.
The /etc/inittab file also contains system initialization script name (rc.sysinit) and and the GETTYs for the virtual consoles. A getty (Get TTY) is a process that waits for a virtual console login and, when a login is successful, connects the logged in user to a terminal session and shell.
All of these startup files are BASH scripts that can be viewed so that you can see exactly what actions are performed during the Startup sequence. You should do so at some point as it is a very interesting exercise. You should never change these scripts, however. There are other, much better ways of modifying the Startup sequence that fall completely within the standard best practices.