Every user on a system may have their own crontab file. The location of the root and user crontab files are system dependant but they are generally below /var/spool/cron.
There is a system-wide /etc/crontab file, the /etc/cron.d directory may contain crontab fragments which are also read and actioned by cron. Some Linux distributions (eg, Red Hat) also have /etc/cron.{hourly,daily,weekly,monthly} which are directories, scripts inside which will be executed every hour/day/week/month, with root privilege.
