Most Linux and Unix-like systems will have a set of system-wide crontab files, typically located at /etc/cron* that are intended for use by cron to run scheduled jobs meant for maintaining the underlying system as a whole.
The /etc/crontab file is a crontab file that allows the specification of a user to run each defined cron job.
The /etc/cron.d, /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, /etc/cron.monthly directories allow systems administrators and configuration management tools to drop individidual files that define jobs into an appropriate schedule. Each file contained within these directories is parsed as its own crontab.