A cron job is the first bit of magic a new webmaster would see in a web hosting account. You tell the server to run a script at scheduled times, and it gets done. Updates happen automatically, data gets filled in automatically, and mails get sent right on schedule. Its like you get your personal robo-servant.
All the modern web hosting control panels like cPanel have beautiful interfaces to add cron jobs as is shown in this WHMCS documentation, and most webmasters will be just happy to use that and be done with it.
However, if you are one of those technology loving webmasters who dream to own a VPS or a dedicated server at some time, and want to know the internal workings of the cron magic, read on.