Automation Approaches: Events/Triggers/Cron [closed]

Question

It has been my experience, when building websites, that most of the logic of a system is executed when user input is accepted, be it via POSTs, GETs etc. I would like to know what processes or methodologies exist in Python (leaning towards using Python), PHP, and Ruby that allow web applications to perform tasks automatically without user input. For instance performing tasks at a certain time or condition or event. I have no experience with, and little understanding of, triggers, events, or cron, and all of the articles I ran across on google during my searches assumed a high familiarity with those concepts. I simply desire a description of ways one could go about handling non-reactive processes in a web application.