OSPF routers have different roles based on their position:
Internal Router: All interfaces in a single area. Has the simplest job.
Backbone Router: Has at least one interface in area .
Area Border Router (ABR): Connects or more areas. Maintains separate databases for each area and summarizes routes between them.
Autonomous System Boundary Router (ASBR): Connects OSPF to external routing domains. Redistributes routes from other protocols (like EIGRP or BGP) into OSPF.
A single router can have multiple roles. An ABR connecting area to area is also a backbone router.