These vary by distribution, so IŽll just list them all (note that this list is by no means comprehensive, so please send me a mail, if you know what the different runlevels do for a distribution not included in this list.
Runlevel 0: halt (shuts down the machine)
Runlevel 1: Single user mode (can be entered by typing Žlinux singleŽ at the LILO: prompt)
Runlevel 2: Multi user text-only, without NFS
Runlevel 3: Multi user text-only, with full networking (this is the default runlevel)
Runlevel 4: Not used
Runlevel 5: Multi user X11, with full networking
Runlevel 6: reboot