Also known as a "cron job," a cron is a process or task that runs periodically on a Unix system. Some examples of crons include syncing the time and date via the Internet every ten minutes, sending an e-mail notice once a week, or backing up certain directories every month.
Crons can be scheduled using the Unix "crontab" command. The jobs are stored in a crontab file and are run by the system at their scheduled times. Or course, if you're not using Unix, you will not need to use a cron, or even know what one is. But, if you've read this far, it certainly is a good term for impressing your friends.