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The cron and anacron daemons
System scheduling is done by the cron daemon, which searches /var/spool/cron for crontab files named after accounts in /etc/passwd; cron also searches for /etc/anacrontab and the files in the /etc/cron.d directory.
cron examines all stored crontabs, checking each command to see if it should be run in the current minute.
Each user can have their own crontab file, but it is often restricted to the root account (see cron.allow and cron.deny in crontab(1)).
anacron runs commands with a frequency specified in days. Unlike cron, it does not assume that the machine is running continuously. Hence, it can be used on machines that aren’t running 24 hours a day, to control regular jobs as daily, weekly, and monthly jobs.