While developing event handlers there are several base classes from which you can inherit.
SPItemEventReciever: One must inherit from this base class if you want the event handler to be executed when the item is added,updated and deleted.
SPListEventReciever: One must inherit from this base class when you want the event handler to execute when the structure of the list is modifed or the content type added or removed from the list.
SPFeatureEventReciever: Inherit this base class if you want your event handler to execute when the feature events such as installation,unstallation,activation and deactivation are executed.
SPWebEventReciever: You can Inherit this base class if you want your event handlers to execute when site is added or removed from the site collection.
SPEmailEventReciever:Inherit this base class for creating a event handler for emails that are send to a Sharepoint list.