Wolfgang Ziegler aka fago did a session about the Rules module at Drupal Con London. With Rules you can define actions which are done when a certain event happens. As such the module provides a GUI to build simple or complex Event -> Condition -> Action sets, which - as fago pointed out - reduces the need of custom clue code. In the tradition of the Views module, it enables site builders to develop functionality without the need to write PHP code. Other modules can provide integration code for Rules, which typically makes certain functionality of the module available as an action or event within Rules. The d7 Commerce project - as an example - got rid of their own Event / Condition / Action thingy and instead just provides Rules integration. Another module that recently got Rules integration in the D7 version isFlag - a module to let user maintain lists of entities (eg a list of favorite nodes). In combination with Rules it can be used to create a "subscribe to a node" feature. User who have subscribed to a node will receive an email notification. There is a Feature which packages this functionality here. Find a full list of modules which provide Rules integration here. Want to learn Rules? Thank God there is Johan Falk! He also maintains a module called Rules Bonus Pack, where he provides some additional Rules functionality.
