In a nutshell, Rules is a visual programming tool. Instead of writing
code as a block of text, you configure the desired behavior via a GUI
that guides you through the setup of "rule sets" which are basically
stored procedures that define conditionals and actions to execute, and
triggering those rulesets from various canned events.
Rules sets can can call each other immediately (watch out for infinite
loops), schedule each other (or themselves) to be called later, etc.