System Administration Guide: Advanced Administration

How the cron Daemon Handles Scheduling

The cron daemon manages the automatic scheduling of crontab commands. The role of the cron daemon is to check the /var/spool/cron/crontab directory for the presence of crontab files, normally every 15 minutes. The cron daemon checks for new crontab files or changes to existing ones, reads the execution times listed within the files, and submits the commands for execution at the proper times.

In much the same way, the cron daemon controls the scheduling of at files, which are stored in the /var/spool/cron/atjobs directory.