init (short for initialization) is the first process started during booting of the computer system. Init is a daemon process that continues running until the system is shut down.
The first thing init does, is reading its initialization file, /etc/inittab
Then init continues to read the /etc/inittab file, which describes how the system should be set up in each run level and sets the default run level.
Available run levels are generally described in /etc/inittab, which is partially shown below:
# inittab This file describes how the INIT process should set up
# the system in a certain run-level.
# Default run level. The run levels are:
# 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)