Both users and the system administrator often need to run specific commands periodically. For example, the system administrator might want to run a command to clean the directories with temporary files (/tmp and /var/tmp) from old files, to keep the disks from filling up, since not all programs clean up after themselves correctly.
The cron service is set up to do this. Each user has a crontab, where he lists the commands he wants to execute and the times they should be executed. The crond daemon takes care of starting the commands when specified.
The at service is similar to cron, but it is once only: the command is executed at the given time, but it is not repeated.