The Rules module allows site administrators to define conditionally executed actions based on occurring event conditions. You can read more about this topic at Wikipedia's ECA-rules article). This can be a complicated subject at first, but the most important take away message going into it is that Rules opens opportunities for site builders and developers to extend Drupal in ways not possible before.
Rules allows you to configure actions to be fired on various events. For example, if a user is creating a new content, the event "Content has been created" occurs. So Rules allows you to react to this event by firing actions. There are a lot of different events available, and contributed modules can even introduce new events. Some more examples of events would be
- a user has logged in
- content has been viewed
- a new comment has been saved
Rules events have nothing to do with calendars. ;)
Rules provides various actions, which let you actually do something. Some examples of actions are
- send an email to a user
- unpublish content
- delete a comment
So the combination of Events and Actions could read like any of these examples:
- When "a user has logged in", "send an email to a user".