Event Receivers are a piece of managed code that responds to SharePoint events when specific triggering actions occur on a SharePoint object.

Event Receivers are triggered by certain events on either a Site collection, Web, List, Field, Item or Workflow. Event Receivers can be attached by either using feature elements definitions or by using the object model (using Feature Receivers). The event handler method that are attached are defined in a separate class Event Receiver class that implements either SPWebEventReceiver, SPListEventReceiver, SPItemEventReceiver or SPEmailEventReceiver.

From https://www.nothingbutsharepoint.com/sites/devwiki/SP2007Dev/Pages/Event%20Receivers.aspx