Crond searches /var/cron/tabs for crontab files which are named after accounts in /etc/passwd; crontabs found are loaded into memory. Crond then 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 (or to the user named in the MAILTO environment variable in the crontab, if such exists).
Additionally, crond checks each minute to see if its spool directory's modtime has changed, and if it has, crond will then examine the modtime on all crontabs and reload those which have changed. Thus crond need not be restarted whenever a crontab file is modified. Note that the Crontab(1) command updates the modtime of the spool directory whenever it changes a crontab.
Paul Vixie, paul@vixie.sf.ca.us