Crond

crond is the name of the daemon which provides the cron scheduling service.

Using crond
If the cron daemon is enabled it will automatically read any created crontabs, the location of which may be different depending on what version of cron is being used and the distribution it is installed on. Crontabs created using the crontab edit command will usually be stored in a sub-directory under /var, the actual files are saved as the user account name that owns them. Instead of creating or editing jobs using crontab their is also the /etc/cron directories which are labeled in the cron.{interval} format, where interval is the preset frequency in which the folder contents will be read. This method offers more convenience and is simpler to use than directly editing a crontab, but offers less control over the specifics of when a job is run.

Different cron daemons
Dillon's cron

fcron

Mellor's cron

Vixie cron