While at is for scheduling a single task to run one time, cron is meant to set up a repetitive schedule. The configuration files for cron are known as tabs. The systemwide cron tab may be found in /etc/crontab, while each of the user cron tabs are found in /var/cron/tabs.
Cron tabs should not be edited directly, but instead should be manipulated using the crontab utility. Note that the crontab utility fires up whatever editor is specified in the EDITOR environment variable, so if you aren't fond of vi you would be well advised to make sure to set EDITOR to ee or nano or whatever else you're comfortable with before firing it up.