What Runlevels Are and How to Use Them
When a Linux system boots, it enters its default runlevel and runs the startup scripts associated with that runlevel. You can also switch between runlevels – for example, there’s a runlevel designed for recovery and maintenance operations.
Traditionally, Linux used System V-style init scripts – while new init systems will eventually obsolete traditional runlevels, they haven’t yet. For example, Ubuntu’s Upstart system still uses traditional System V-style scripts.