init

The single most important service in a UNIX system is provided by init . init  is started as the first process of every UNIX system, as the last thing the kernel does when it boots. When init  starts, it continues the boot process by doing various startup chores (checking and mounting filesystems, starting daemons, etc).      

The exact list of things that init  does depends on which flavor it is; there are several to choose from. init  usually provides the concept of single user mode, in which no one can log in and root  uses a shell at the console; the usual mode is called multiuser mode. Some flavors generalize this as run levels; single and multiuser modes are considered to be two run levels, and there can be additional ones as well, for example, to run X on the console.        

In normal operation, init  makes sure getty s are working (to allow users to log in), and to adopt orphan processes (processes whose parent has died; in UNIX all processes must be in a single tree, so orphans must be adopted).

When the system is shut down, it is init  that is in charge of killing all other processes, unmounting all filesystems and stopping the processor, along with anything else it has been configured to do.