A List Event Receiver, simply put is code that you (the developer) can choose to "attach" to one or more lists and that code will (generally) run when either you add, delete, or update items on a list.
List Item Event Receivers are important as they provide you the opportunity to provide some custom error handling or validation logic, and even cancel/prevent the add/update/delete from taking place. The functionality is provided by inheriting from the
SPItemEventReceiver class and overriding one of the following key methods :
You will note from this list the term used - "Before" and "After" events. The before events end in "ing", are trapped before the, say update, ...