Why LSA type 3 is called summary LSA
As you are already aware, In OSPF routers are divided into areas. Routers within the area know all the information about the fellow router through LSA 1 and 2. This database is fairly detailed as the router can interpret the entire topology of the area using the LSAs.
On the contrary the only infomation available to the router for router in other area is:
This router is belongs to Area XX and I can reach this router via ABR router YY.
The router is not aware how the topology is in that area or if the ABR is directly connected to the other router.
This is because when an ABR sends LSA type 3, It only sends the necessary information necessary to enable communication. In a way it summarises the routes. Hence the name summary LSA
**From what I've learnt, ABR summarises the area but when I get into the database of a router in a particular area, I see there is info about all the routers present in another area rather than the network summarisation. So what does LSA type 3 actually contain sent by the ABR?*
The information about all the routers have to be there, otherwise how can we ensure connectivity between all users in the network.
As to what exactly does LSA 3 contain. You can check a wireshark packet at packetlife.com or create a network yourself on GNS3.
But for understanding purposes, what the ABR packet says is: I can reach this network or this address and the cost will be XX. But it does not tell how will it reach the network. Or where is the network.
when does the ABR send LSA type 3
A router will be an ABR when it is connected to area 0 and some other area.
So it will have OSPF running on multiple interfaces one of them will be in area 0. Other interfaces can belong to any area.
Now consider only area 0. In area 0 the ABR will send LSA 1 and 2 like all other routers to have the complete information of the topology.
Let the other area be area 1, On the interface connected to area 1, the ABR will sned and recieve LSA 1 and 2 to get the entire view of area 1.
Now the summarise area 0 and send 1 LSA type 3 to area 1
Similarly it will summarise area 1 and send one LSA type 3 in area 0.
For a better understanding see this video on OSPF areas here
To clarify stuff, by having a topology view I mean a router will know where an interface of a given router is connected to.