System Administration Guide, Volume 2

How the cron Daemon Handles Scheduling

The cron daemon handles the automatic scheduling of crontab commands. Its function is to check the /var/spool/cron/crontab directory for the presence of crontab files, normally every 15 minutes. It 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.