OSPF routers can be divided into four categories according to the different locations in the AS.
Routers within the region: all the interfaces of this kind of router belong to one OSPF area.
Area Border Router: This kind of router can simultaneously belong to two or more areas, but one must be the backbone area. ABR is used to connect the backbone area and non-backbone area. The connection can be either a physical connection or a logical connection.
Backbone router: This kind of router has at least one interface belongs to backbone area. Therefore, all the ABR routers and routers located in area0 are backbone routers.
Autonomous system boundary router: Router exchanging routing information with other AS is called ASBR. ASBR does not necessarily located at the boundary of the AS, it may be a IR or ABR