Content Organizer: is a feature in SharePoint that can be used to move the documents/content from one common location to any other location (Site-collection/Site/Library/Folder within library) without manual user intervention. 

Drop off Library: After activating Content Organizer feature, one library (Drop off Library) gets created. This library is used to upload documents via manual uploads, workflows, the object model, etc. 

• Any content that is received by the Drop off Library can be routed to alternate locations without user intervention. The location that the content is routed to is determined by rules and settings done in Content Organizer