Internal Routers are routers whose interfaces all belong to the same area. These routers have a single link state database.
Area Border Routers (ABRs) connect one or more areas to the backbone and act as a gateway for interarea traffic. An ABR always has at least one interface that belongs to the backbone, and must maintain a separate link state database for each of its connected areas. For this reason, ABRs often have more memory and perhaps more powerful processors than internal routers. An ABR will summarize the topological information of its attached areas into the backbone, which will then propagate the summary information to the other areas.
Backbone Routers are routers with at least one interface attached to the backbone. An Internal Router whose interfaces all belong to area 0 is also a Backbone Router.
Autonomous System Boundary Routers (ASBRs) are gateways for external traffic, injecting routes into the OSPF domain that were learned (redistributed) from some other protocol, such as the BGP and EIGRP processes. An ASBR can be located anywhere within the OSPF autonomous
system; it may be an Internal, Backbone , or ABR.