Cron is a daemon that executes scheduled commands.
Cron reads /etc/crontab.
Cron wakes up every minute, examining all stored crontabs, checking each command to see if it should be run in the current minute.
When executing commands, any output is mailed to the owner of the crontab.
You should use absolute path names for commands like /bin/ls. This is to insure you call the correct command.
# Minute Hour Day of Month Month Day of Week Command
# (0-59) (0-23) (1-31) (1-12 or Jan-Dec) (0-6 or Sun-Sat)
0 2 12 * * /usr/bin/find
0 0 1 1 * - Run once a year
0 0 1 * * - Run once a month
0 0 * * 0 - Run once a week
0 0 * * * - Run once a day
0 * * * * - Run once an hour