Ok, so this is sort of basic stuff, but I think the diagram helps make conceptualizing it easier. Why should you be on Twitter you ask? Well – because one person says something and then all his/her followers see that message. Then if his/her followers respond (@reply) or syndicate (retweet) the comment, all of their followers see the message. It’s a ripple affect, whereby your message can be carried instantly to a large group of people. What’s more, by virtue of the relationships (ie. I only follow people/things that matter to me, and presumably people only follow me because I talk about things that matter to them) the content is being spread to the most interested and relevant audiences.
That’s enough learning for today. . . but if you still have questions, just ask!