Edit existing crontab file, or make new one:

crontab -e

List of cronjobs , display crontab file contents:

crontab -l

Removes the crontab file:

crontab -r

Time format in crontab

*****command
min (0-59)hour (0-23)day of the month (1-31)month (1-12)day of the week (0-6)
Sunday = 0
* is a wildcard, meaning any value from the given range

Examples:

@reboot perl /usr/local/bin/dnsexit/ipUpdate.pl

Executes script ipUpdate.pl on every computer restart

0 11 * * * rsync -rav --delete /source.dir/ /target.dir

Rsync(s) every day at 11h