cron, at and anacron are system processes used for periodic command execution.
cron and at differ in one respect. cron is based on an absolute timetable whereas at is relative. For example, a cron entry would execute a command every Thursday in February. An at entry would run a specific command exactly 23 hours and 17 minutes from now.
It's also worth mentioning anacron here, as well. anacron allows tasks to be scheduled and performed when the system is not running 24 hours a day, 7 days a week.
Debian comes with several housekeeping cron entries enabled by default. Each of these entries may be viewed by logging in as the appropriate user (likely root) and running the crontab command.
Any user may view the system's scheduled tasks by looking at the /etc/crontab file:
$ cat /etc/crontab
Debian does not come with any at entries enabled on a standard installation. To view any at entries that you have submitted, use the atq command.