Let's consider a host A in an autonomous system AS100. Suppose it wants to communicate with host B in another AS, say, AS200. In AS100 there are several routers. Within one AS, the rotuing protocols used are Internal gateway protocols such as OSPF or RIP.
AS100 needs to know the path through other AS-es to AS200, so the edge routers in AS100 are BGP routers, BGP is an external gateway protocol used for routing between autonomous systems.
If AS100 is connected to, say, two ASes (it has two eBGP routers), how does host A inside AS100 know which BGP router to talk to, in order to communicate with host B in AS200? Host A tells its router that it wants to communicate with host B. Now, that router would need to know the path to host B. But host B is outside AS100.
Do eBGP routers send their routing tables to all routers (both BGP and IGP) in one AS? I mean, 'normal' routers inside AS have to know a lot, they have to choose the appropriate border router so that host A can calk to host B.
If all routers know the same thing, then why would anyone need iBGP? Generally BGP routing tables are big, so it's probably a false statement, but I'm not sure how could host A know the right path if its neighbour routers didn't know all the routes the border routers have.