Asyncronous/synchronoug communication: how do the poker clients work?

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Does anybody know the answer to this question? Are poker sites using sockets? Web services? UDI? Asynchronous/synchronous commuincation?

I'm working on a project that requires similar types of commnication, just not as frequent and realtime and was wondering if anybody had any insight.

Also, anyone know why some sites would be blocked by my firewall and others wouldn't.

Thanks in advance.