OSPF Router Types
There are four types of OSPF routers which are determined by a router's function and/or location within an OSPF area:
Internal (IR) -- all OSPF interfaces must belong to the same OSPF area.
Backbone -- at least one OSPF interface must belong to area 0 (backbone area)
Area Border Router (ABR) -- at least one OSPF interface must belong to area 0 (backbone area) and at least one OSPF interface must belong to a non-backbone (area 0) area.
Autonomous System Boundry Router (ASBR) -- an OSPF router that performs route injection (redistribution) from another route source (RIP, EIGRP, IS-IS, BGP, another OSPF process, etc.).
You can use the show ip ospf EXEC command to identify which OSPF type(s) your device is currently configured to be.