Remote event receivers work similarly to regular event receivers except that remote event receivers handle events that occur when an app for SharePoint is on a different system from its host web application.
Event receivers in earlier versions of SharePoint primarily ran on the SharePoint Server to handle events that occurred on SharePoint objects such as list, libraries, site etc.
In SharePoint 2013, a new concept, Remote Event Receivers, has been introduced, where the event generated by a SharePoint app could be listened to and handled by the SharePoint Server.
http://www.c-sharpcorner.com/UploadFile/3a164c/remote-event-receivers-in-sharepoint-2013/
