Cron runs things in your crontab.

The format is something like date parts, command, user.

Anacron is like cron, except that it uses timestamps to work out if a thing was supposed to run in the past, but didn't (e.g. the machine was off).

In Debian, anacron can be installed as an apt package which puts in a systemd service. It will have a default file

Debian has some folders /etc/cron/cron.hourly, cron.daily and cron.monthly.

Debian's default /etc/crontab attempts to run these, unless anacron is installed, in which case it will be caught be /etc/anacrontab instead.

The at command lets you schedule one thing to happen at a particular time.

Some examples:

at now + 3 minutes
at noon

echo "notify-send 'oy'" > command.txt
at now + 1 minute -f command.txt

atq lists the scheduled jobs

atrm n deletes job n in the schedule