The Tiny Book of Rules
This is Rules
Rules is a module usually used to automate actions on your Drupal site. Rules can react on events occurring on your site – such as a user logging in or a node being created – and perform customized follow-up actions such as redirecting to a certain page or setting field values. Optionally, Rules can evaluate conditions before executing any actions.
Rules is also a framework to use by other modules, allowing them to evaluate user-configured Rules components, such as action and condition sets, or expose data about variables and entities to other parts of Drupal.