It's synchronous! That's the “real-time” aspect that's touted. This is in strong contrast to the highly asynchronous nature of email and BBS/forum messages. (And that asynchronous property is highly useful, by the way).
It's UDP! Twitter messages are like packets broadcast out to the world. There's no ACK. You don't care if they aren't received (mostly). However, just like a broadcast time service has useful information that a computer can pick up when it needs to, Twitter is a highly useful stream of information you can dip into.