On March 5, 2010, the microblogging service known as Twitter (www.twitter.com) published its 10 billionth tweet. The company claimed that the short text messages were now flowing at the rate of about 600 per second. Not bad for a service not far from its fourth birthday. Today, people use Twitter for everything from publishing personal musings to pushing products, tracking trends to circumventing the censorship of repressive governments. The Twitter stream is flowing fast these days; if you're ready to jump in, here's what you need to know to enjoy the swim.