If you're not up on Unix lingo, a cron job is a scheduled action that occurs at preset intervals on your server. For instance, Magento can create a new sitemap for your store according to the time interval you configure in the backend.
What is confounding to many new to Magento is that configuring cron intervals for various Magento functions doesn't actually cause anything to happen. The reason is that your server must still be told to run the configured tasks.
Cron jobs are configured by using what are called crontabs. These are expressions that dictate how often the server is to run the particular task.
There are a few inherent functions included with Magento that can be run periodically, including: