crontab is a scheduling system available on most linux distributions
Sometimes users will create cron jobs that will slow a server down. Access to cron can easily be restricted to all users but the administrator by editing /etc/cron.allow
If, for example, root and only one person (with username user) should be the only ones able to use cron just put root on one line and user on another in /etc/cron.allow.
Note: If cron.allow exists only users in the file will be able to use cron.