So first of all, what is Twitter? The best analogy I can think of is that it's like CB radio, but for text messaging. You basically answer the question "What are you doing right now?" at different times throughout the day, and the messages that you send have to be short -- 140 characters or less. So some examples of what you might say are:

Having drinks at the Horseshoe Bar with my co-workers.
zappos 2 minutes ago
or

Eating sushi.
zappos 1 minute ago
or

About to fly to New York from Vegas.
zappos less than a minute ago

You send an SMS text message to Twitter with your note, and your message will be automatically broadcast (like CB radio) to whoever is choosing to follow you (your friends). If people don't care what you're doing, they won't follow you, so don't worry about sending out trivial messages.

At first, it will seem really weird and unnatural for you to do this, but just trust me on this one. You will find that it's actually a really good way to stay in touch with all your friends and know what's going on in their lives.
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Tony really explains what Twitter is...and how to go about doing it. Check it out here>>