How does twitter work?

[Tim]

Imagine it as a wall. You add friends that can see your wall and they add you so you can post on their wall.
You can tweet (send a short message) to update your status and let all your friends know what you are upto. Thats the whole point. To keep people in the know about what you are doing.
The comments that didnt seem to make sense is when someone directly replies from one wall back to you so you would only understand what they are on about as you posted on their wall and they have now replied on yours.