From LQWiki

• dmesg - display bootup messages.
• halt - halt the system. (not recommened - use shutdown -h now instead)
• init - set runlevel, or define processes that are begun on a specific runlevel.
• poweroff - power the system off.
• rdev - display system startup configuration info.
• reboot - reboot the system.
• runlevel - show the current system runlevel.
• shutdown - shutdown the system.
• swapoff - disable the paging hardware.
• swapon - enable the paging hardware.
• sync - write buffered memory out to disk.
• telinit - move the system to a new runlevel.