In the e-commerce flow of Magento, there are a lot of events that happen when a visitor buys something from your shop. He or she adds a product to the shopping cart, selects a payment method, logs in, and so on.
Magento dispatches these events and you have the possibility to hook in to an event to do your stuff. It's like hooking in to a click event in JavaScript.
The Observer design pattern is used to implement the event handling system. An event happens in the installation, and the configuration will call the right function from an observer class to execute.
The cronjob system in Magento is built on the same design pattern. Cronjobs are configured in the configuration. When the cron runs, Magento will check the timeframes and execute the jobs that match the configuration.