It's a SAN, a Storage Area Network.
Each site is stored on a server (either its own or someone else's) The internet is just the interconnection between all these servers.
Servers
Their like, where you want to play. Like, it's separate worlds.
You can talk to people on separate servers and channels. So its very convenient.
Servers
Private servers and live servers have nothing to do with each other - the two accounts are separate, and nothing earned in a private server exists on a live server.
You can't. Unfortunately, Blizzard considers the US servers and users completely separate from the EU realms and servers, and you cannot transfer characters across, and the software itself doesn't work either. You have to get a new membership and account if you wish to play in both EU and US.
Both the mindcrack and mindcrack fan servers are white-listed servers. currently mindcrack is only sending private invitations to a select few and the mindcrack fan server's residents are holding separate contests to join.
Europeans are capable of playing on American (US) servers just as Americans are capable of playing on European (Oceanic) servers. The only difference between them, other than the culture of those on the server, is the physical location of the servers them selves. The greater the distance, the more likely you are to experience latency (lag) issues. It should be noted that for Americans to play on European servers (and vice-a-verse) you will need the appropriate account. If you want to play on American servers, you need an American account. It does not matter where in the world you are accessing the servers from, you can not access one regions servers if you do not have the appropriate regions account. The two regions are completely separate.
It probably wont work if you try connecting to North American/Oceanic servers because it is a separate client. You can use it to connect to European servers while you are in America, but there might be a bit of a problem with lag.
There are mail servers, FTP servers, Web servers, DNS servers, DHCP Servers, those are the most common types
Web Servers Mail Servers Application Servers