Difference between Cron and Crontab?
cron is the general name for the service that runs scheduled actions. crond is the name of the daemon that runs in the background and reads crontab files. A crontab is a file containing jobs in the format
minute hour day-of-month month day-of-week commandcrontabs are normally stored by the system in /var/spool/<username>/crontab. These files are not meant to be edited directly. You can use the crontab command to invoke a text editor (what you have defined for the EDITOR env variable) to modify a crontab file.
There are various implementations of cron. Commonly there will be per-user crontab files (accessed with the command crontab -e) as well as system crontabs in /etc/cron.daily, /etc/cron.hourly, etc.
In your first example you are scheduling a job via a crontab. In your second example you're using the at command to queue a job for later execution.