The cron daemon reads crontab files (usually located in /var/spool/cron/crontabs) to determine when to execute cron jobs. Crontab files can be installed on a per-user basis, and cron jobs are run under the uid of the user who installed them.
Crontab files should generally not be edited directly. A utility called crontab is used for managing crontab files. crontab -l will show the contents of a user's crontab file. crontab -r will remove the user's crontab file. crontab -e will invoke an editor on the user's crontab file (the EDITOR environment variable can be used to select the editor of your choice, which usually defaults to vi). crontab knows how to signal cron to re-read modified crontabs.
It is usually also possible to restrict cron access to specific users by placing usernames in the files /etc/cron.allow or /etc/cron.deny files. If cron.allow exists then only the users listed in it will be permitted to use crontab. If cron.deny exists then any users listed in it will not be allowed to use crontab. Administrators can thereby allow only authorized users to install cron jobs or deny access to uses who abuse cron.