Cron is a very usefull service in Linux systems that allows periodical execution of programs.
Terms
The program that runs other programs periodically is called "cron".
The configuration file, that tells cron which programs should be run
periodically is called "crontab" and located in /etc/crontab.
One line in that file is called "cronjob".
But "crontab" is also a command that lets you add cronjobs.
Example
The following entry in crontab means that uamt.py is executed
every day at 3:30 am:
30 3 * * * /home/mthoma/uamt.py
You can check if everything went fine with
mthoma@i13srv30:~> crontab -l
# DO NOT EDIT THIS FILE - edit the master and reinstall.
# (/tmp/crontab.DEZBGc installed on Sat Jan 4 14:35:03 2014)
# (Cronie version 4.2)
30 3 * * * /home/mthoma/uamt.py