a shorthand way to tell the system to get into a certain state of configuration with respects to what services are running (daemons, see below), whether to allow remote logins, what filesystems are mounted read-only or read-write, etc
Specifies which system services are started up when Linux boots. Different runlevels start the services necessary for networking, multiple users, and graphical login sessions. The default runlevel for a Red Hat Linux graphical workstation is 5.
The term runlevel refers to a mode of operation in one of the computer operating systems that implement Unix System V-style initialization. Conventionally, seven runlevels exist, numbered from zero to six.