Creating or adding extra customised features to the sharepoint components like
SharepointItems,List and WebEventReceivers is called "Features in Sharepoint/moss 2007".
usually developer create the customised action by deriving the event handler class of the sharepoints ie.SPItemEventReceiver,SPListEventReceiver and SPWebEventReceiver and attach to the sharepoint site.
Administrator installing the features and activate them on the site or web.
Synchronous Event:
Events that activate before the action occur
Example : FieldAdding event of SPListEventReceiver
Asynchronous Event
Events that activate before the action occur
Example : FiledAdded event of SPListEventReceiver