Backbone router ( OSPF )

These are routers that are part of the OSPF backbone. By definition, this includes all area border routers, since those routers pass routing information between areas. However, a backbone router may also be a router that connects only to other backbone (or area border) routers, and is therefore not part of any area (other than Area 0).

To summarize: an area border router is always also a backbone router, but a backbone router is not necessarily an area border router.