cron, anacron and at 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.
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 files /etc/crontab and /etc/cron.*/
crontab(1) - tool to maintain crontab files for individual users
crontab(5) - syntax of tables for driving cron
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 (It's installed by default by Debian-Installer on laptops, and with the desktops task).
Any user may view the system's scheduled tasks by looking at the /etc/anacrontab file:
$ cat /etc/anacrontab
anacrontab(5) - configuration file for anacron
anacron(8) - runs commands periodically
at (and batch) execute a command at a later time (once). 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.