Similar to how Microsoft Windows has “safe” and “command prompt only” modes you can boot the operating system into, the Linux operating system has different modes of operation — or “runlevels” — it may run in. Unlike Windows, however, you can change Linux runlevels on the fly.
When you boot a computer running Linux, it will boot into a default runlevel (this is usually level 3 or level 5). There are six different runlevels most Linux distributions use (with System V init - like Red Hat, or Mandrake)