You're talking about the master server, right? It's run by Epic, and when someone starts a server of their own, the machine contacts the master server, which in turn advertises it to other players. Not all servers report to the master server though, in which case a person would have to know the IP of the server in order to play on it.
http://unreal.epicgames.com probably has answers for all the technical questions you have about Unreal/UT.
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[This message has been edited by -Axiom- (edited March 10, 2001).]