6.5.4 Runlevel usage

When installed, most Debian packages configure their services to run in runlevels 2 through 5. Thus, there are no differences between runlevels 2, 3, 4 and 5 on a Debian system that has not been customized; Debian leaves it up to the local administrator to customize runlevels as described in Customizing runlevels, Section 2.4.3. This differs from the way runlevels are used by some other popular GNU/Linux distributions. One change you may want to make is to disable xdm or gdm in runlevel 2 so that the X display manager is not started at the end of the boot sequence; you can then start it by switching to runlevel 3.

For more information about runlevels see Runlevels, Section 2.4.2.