A cron job or crontab is a Unix command that a program or script that runs at a specified time . Cronjobs are used in Unix-like systems like Linux, BSD, and Macintosh. The word "cron" is derived from the word chronograph, which is a kind of stopwatch. A cron job on a system service is called a daemon. These are mostly server applications.
Cronjobs are often used to check e-mail, sending mail or checking for updates. If a Cronjob on a (web) server is used, this is often a database backup, or a PHP or perl script to execute a command.