OSPF Area Type
The backbone area (Area 0) is the core of an OSPF network. All other areas are connected to it and all traffic between areas must traverse it. All routing between areas is distributed through the backbone area. While all other OSPF areas must connect to the backbone area, this connection doesn’t need to be direct and can be made through a virtual link.
Normal OSPF Area
In a normal OSPF area there are no restrictions; the area can carry all types of routes.
Stub OSPF Area
A stub area does not receive routes from other autonomous systems. Routing from the stub area is performed through the default route to the backbone area.
The Not So Stubby Area (NSSA) is a type of stub area that can import external routes, with some limited exceptions.