cron is a unix system of starting processes at defined intervals.
Over the years (unix dates back many decades), cron has a evolved a bit and the capabilities have hence also expanded.
Essentially there is a table (mostly nowadays per user) that defines what needs to be started when. It's called a crontab.
Most unices allow a user to list their cron entries you have with the command $ crontab -l most also allow the user to edit their entries with the command $ crontab -e It'll use your favorite editor if you set any, vi other wise - which you might dislike if you're not an old time unix user. (type :q! to quit in that case and set the appropriate editor settings)
It's without a doubt the very best way to start something recurring if you have shell access to a user on a unix machine. It's highly reliable and rather flexible.
The format of a crontab line is 6 fields (separated by spaces or tabs): col1: minute col2: hour col3: day of month col4: month col5: day of week col6: the command (can include spaces all it wants)
Uncommon unix machines (essentially unix v7 machines) had only one global crontab and included a 7th field (before the 6th) that told cron to run the command as which user)
A star in one of the fields means "every"
e.g. 0 0 * * 0 /path/command
will start every sunday a midnight (0 is sunday, 1 is monday, ... 6 is saturday, 7 is sunday as well) (on every month every day of the month)
more modern variants also support shorthands like @weekly (which is defined to mean "0 0 * * 0") and/or things like */5 (every 5) and/or ranges like 4-8 (at 4, 5, 6, 7 and 8) and/or en enumeration like 1,2,3,5,8,13
The output of the command is email locally to the user running the command (so if your email is configured properly you get the output in an email).
root's cron should have a number of system maintenance commands in it for inspiration.
There is a process running on every unix system called "cron" that reads the crontab(s) and spawns the needed commands at the right time.
That's in a nutshell and non specific to the different flavours of unix.