crontab is used for executing scheduled processes. For example, you can run database backup everyday using schell script.
Above command list the all available crontab command.
This command can be used to edit crontab command fro your preference.
Sample crontab command is shown below. This script.sh script is executed every day at midnight.
0 0 means zero minute at zero hours(midnight).
0 0 * * * /home/sujith/script.sh