The Sysvinit package contains the pidof, last, lastb, mesg, utmpdump, wall, halt, init, killall5, poweroff, reboot, runlevel, shutdown, sulogin and telinit programs.
Pidof finds the process id's (pids) of the named programs and prints those id's on standard output.
last searches back through the file /var/log/wtmp (or the file designated by the -f flag) and displays a list of all users logged in (and out) since that file was created.
lastb is the same as last, except that by default it shows a log of the file /var/log/btmp, which contains all the bad login attempts.
Mesg controls the access to the users terminal by others. It's typically used to allow or disallow other users to write to his terminal.
utmpdumps prints the content of a file (usually /var/run/utmp) on standard output in a user friendly format.
Wall sends a message to everybody logged in with their mesg permission set to yes.
Halt notes that the system is being brought down in the file /var/log/wtmp, and then either tells the kernel to halt, reboot or poweroff the system. If halt or reboot is called when the system is not in runlevel 0 or 6, shutdown will be invoked instead (with the flag -h or -r).
Init is the parent of all processes. Its primary role is to create processes from a script stored in the file /etc/inittab. This file usually has entries which cause init to spawn gettys on each line that users can log in. It also controls autonomous processes required by any particular system.
killall5 is the SystemV killall command. It sends a signal to all processes except the processes in its own session, so it won't kill the shell that is running the script it was called from.
poweroff is equivalent to shutdown -h -p now. It halts the computer and switches off the computer (when using an APM compliant BIOS and APM is enabled in the kernel).
reboot is equivalent to shutdown -r now. It reboots the computer.
Runlevel reads the system utmp file (typically /var/run/utmp) to locate the runlevel record, and then prints the previous and current system runlevel on its standard output, separated by a single space.
shutdown brings the system down in a secure way. All logged-in users are notified that the system is going down, and login is blocked.
sulogin is invoked by init when the system goes into single user mode (this is done through an entry in /etc/inittab). Init also tries to execute sulogin when it is passed the -b flag from the boot loader (eg, LILO).
telinit sends appropriate signals to init, telling it which runlevel to change to.