What is the difference between the commands wall, sleep and killall? If an example is given on each it would help me to understand better.

sleep:
This command is used in UNIX operating system to suspend execution of the system for the specified time limit mentioned in it as parameter.
wall:
wall is used for sending messages to users of UNIX operating system
killall:
The command killall is used to kill processes which are active. The main aspect in this is killall command can be issued only by the super user.

Additional Info about the Messaging command,
Wall - sends messages to all users who are logged in.
This command displays the contents of file or, by default, its standard input, on the terminals of all currently logged in users.

Only the super-user can write on the terminals of users who have chosen to deny messages or are using a program which automatically denies messages.
Reading from a file is refused when the invoker is not superuser and the program is suid or sgid.

Another command to send messages is write command.
The write utility allows to communicate with other users (user or terminal id mentioned) by copying lines from your terminal.
write user [tty]

You can prevent users (other than the super-user) from messaging you with the mesg command.

*** Innila ***