How RH Linux Boots :: Runlevel

The runlevel is an abstraction represented by a single character, generally a digit from 0 to 6, but higher numbers and even lettters, such as S, s, a, b, or c, are allowed.  It is used to coordinate the shutting down and starting up of specific processes.

Typical runlevels are:

0 -  shutdown [reserved]
1 - single user mode [reserved]
5 - normal: multiple user, X
6 - reboot [reserved]

The runlevel can be set as a boot time option, from within /etc/inittab, or by /sbin/telinit.

Once set, the runlevel is globally available.