it's just a way of sharing short messages with your friends and others

twitter.com says: "Twitter has grown into a real-time short messaging service that works over multiple networks and devices.", which isn't saying much.

In short, Twitter is a way to concisely describe what you're doing in your day to day life knowing that your friends, and if you like, the public, will see your updates. Likewise you can watch your friends' daily events and learn more about what's going on right now.