When a system is finished booting, the kernel spawns init as process 1. Init then boots the rest of the system by means of the rc (runcommand) scripts which eventually spawn getty processes so that you can log in. If you boot in single user mode, init will spawn a bourne shell for you and leaves the system in this state so that you can do maintenance. Init is also inherited as the parent process of processes that have been orphaned. It's duty thus is to reap zombie processes of those that died.
$ man 8 init
to understand the init process of your operating system.