A runlevel is a software configuration of the system which allows only a selected group of processes to exist. The processes spawned by init process for each of these runlevels are defined in the /etc/inittab file. Runlevels 0, 1, and 6 are reserved. Other runlevels are dependent on how your particular distribution has defined them, and they vary significantly between distributions.

Most users run X from runlevels 3 or 5 (for linux?). Runlevel 3 places your system in multi-user mode with full networking capabilities. The machine will boot to a text-based login prompt with all necessary preconfigured services started. Most servers are run in runlevel 3, as X is not necessary to provide any services utilized by most users. Runlevel 5 is similar to 3, except that it automatically starts X and provides a graphical login screen. Many workstation users prefer this method, because it never forces them to see a command prompt.

You can add commands that must be executed in appropriate runlevel in some file/under some directory (e.g. /etc/rc.d/rc5.d)