Understand Open Shortest Path First (ospf) Design Guide

Areas and Border Routers
As previously mentioned, OSPF uses floods to exchange link-state updates between routers. Any
change in routing information is flooded to all routers in the network.
Areas are introduced to put a boundary on the explosion of link-state updates. Floods and
calculation of the Dijkstra algorithm on a router is limited to changes within an area.
All routers within an area have the exact link-state database. Routers that belong to multiple
areas, and connect these areas to the backbone area are called area border routers (ABR).
ABRs must therefore maintain information that describes the backbone areas and other attached
areas.
An area is interface specific. A router that has all of its interfaces within the same area is called an
internal router (IR).

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A router that has interfaces in multiple areas is called an area border router (ABR).
Routers that act as gateways (redistribution) between OSPF and other routing protocols (IGRP,
EIGRP, IS-IS, RIP, BGP, Static) or other instances of the OSPF routing process are called
autonomous system boundary router (ASBR). Any router can be an ABR or an ASBR.
Link-State Packets
There are different types of Link State Packets, those are what you normally see in an OSPF
database (Appendix A and illustrated here). 
The router links are an indication of the state of the interfaces on a router in a certain designated
area. Each router generates a router link for all of its interfaces.
Summary links are generated by ABRs; this is how network reachability information is
disseminated between areas.
Normally, all information is injected into the backbone (area 0) and in turn the backbone passes it
on to other areas.
ABRs also propagate the reachability of the ASBR. This is how routers know how to get to
external routes in other ASs.

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Network Links are generated by a Designated Router (DR) on a segment (DRs are discussed
later). 
This information is an indication of all routers connected to a particular multi-access segment such
as Ethernet, Token Ring and FDDI (NBMA also).
External Links are an indication of networks outside of the AS. These networks are injected into
OSPF via redistribution. The ASBR injects these routes into an autonomous system.