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Different Linux distributions use runlevels differently. One commonality is that runlevels 0 and 6 are reserved for shutdown and runlevel 1 is single-user mode. Single-user mode is generally used for doing system repairs. On Red Hat systems runlevel 2 is mult-user without networking, runlevel 3 is multi-user with networking, runlevel 4 is for custom configuration and runlevel 5 is multi-user with graphical login.
There are a at least three tools for configuring and managing runlevels on RedHat Linux systems. The graphical interface tool, redhat-config-services can be reached from Menu -> System Settings -> Server Settings -> Services. The default it to edit the currently running Runlevel. By choosing the Edit Runlevel tab, you can edit runlevel 3, 4 and 5. You can run man redhat-config-services for a synopsis on this program.
There is an ncurses based tool, for configuring runlevels. It too, will default to the current runlevel. However, according to the man page, you can edit different runlevels by calling it with the -level argument. Calling man ntsysv will give you more information for running the runlevel editor.
There is also a general purpose command line tool for managing runlevels, /sbin/chkconfig for querying and updating the runlevel system. Reading about chkconfig reveals some of the details about the mysterious numbers (S40, etc) in the scripts. Checkout also the chkconfig data that is embedded in the comments at the beginning of /etc/rc.d/init.d/ scripts (the ones that are "managed by chkconfig"). - /sbin/init (or /sbin/telinit) for changing runlevel, re-reading the inittab file, or activating "ondemand" processes.
See man init for details.