A runlevel is a configuration of the system software that only allows certain selected processes to exist. Allowed processes are defined, for each runlevel, in the file /etc/inittab. There are eight defined runlevels: 0, 1, 2, 3, 4, 5, 6, S. You can create your own runlevel too. For a more detailed description about runlevels, please refer to "The startup files: init sysv".
Booting into a different runlevel can help you solve certain problems, for example: suppose you have made a change into your X configuration that has rendered it unusable, and you boot into it by default. If so, you can temporarily boot into a console, fix the error and reboot into X. Let's see how to do that.
By default GNU/Linux either boots to runlevel 3 (the console) or to runlevel 5 (X). The default runlevel is defined in the file /etc/inittab. Look for an entry like id:3:initdefault: (if your system starts in a console) or one like id:5:initdefault: (if your system starts in X).
If you want to boot into a runlevel other than the one defined in /etc/inittab, you have to define that runlevel on the bootprompt. Under LILO, type linux init 3 to boot into the console or linux init 5 to boot into X. Under grub, press the E key twice, add init 3 to boot into the console, or init 5 to boot into X, press the ENTER key and then the B key to boot.