System V Init refers to the way the start up and shutdown of an operating system ( mostly linux and such right now. correct me if i'm wrong) and it's services (daemons) is handled. The flexibility and control are obtained by the use of runlevels - different sets of configurations.
The runlevel config is typically specified in /etc/rc.d/rc0.d/ (/etc/rc.d/rc1.d/, /etc/rc.d/rc2.d/ etc) directory where rc0.d is replaced by any runlevel number (ranging from 0-6). The files in these directories have the following format:
lrwxrwxrwx   1 root     root           15 Jan  9  2000 K45named -> ../init.d/named
lrwxrwxrwx   1 root    root           16 Jan  9  2000 S55routed -> ../init.d/routed
Every entry is in fact, a symbolic link to a respective start up script. Each file beginning with "S" will envoke the corresponding script with a "start" command (eg: ../init.d/routed start from the previous example). On the other hand "K" entries will send the kill command ( or stop).
  • To remove a service a from runlevel, remove a corresponding symlink; or alternatively rename the symlink to something else (like making S55routed into s55routed - thanks to say.).
  • To add a service to a runlevel, add a symlink to an appropriate start up script.
  • To change the current runlevel, use init run_level_number
  • To change the default boottime runlevel, look in /etc/inittab

List of Runlevels

from Red Hat 7 /etc/inittab
0 - halt (Do NOT set initdefault to this)
1 - Single user mode
2 - Multiuser, without NFS (The same as 3, if you do not have networking)
3 - Full multiuser mode
4 - unused
5 - X11
6 - reboot (Do NOT set initdefault to this)

Note 1: Credits go to melodrame
Note 2: Based on

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