more important than the president/sandy

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Peer-to-peer infrastructure (p2p). Unlike a Client-Server system where clients (chat users) send messages to the server which then sends the message to the recipient-client, a p2p system like Skype uses the server just for account information. If you send someone a message on Skype, the program asks the Skype server where to send the message and then sends it "directly" to the recipient - thus, if he's not online, it cannot be send, as there is no message server that would store the messages until delivery.
(May not be exactly correct, but that's the idea) :<

p2p chat has pros and cons.