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
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.