Cron is a daemon in *NIX systems that take care of scheduled execution of programs. It is used mainly to automate administration tasks.
Cron reads a file called crontab, and decides what jobs need to be run and at what time.
Traditionally there was only one crontab, kept as /etc/crontab (or something similiar); in case of Vixie Cron, popular version of Cron that is used in Linux and other systems, there is a more elegant approach of using crontab pools where each user can add their own jobs with "crontab -e" command, without the need for root user to edit crontab.
Not to be confused with Crom. See cron jobs, crontab file format