How does twitter work?

[rosebush hater]

I know that it is some kind of social website, like facebook or whatever, but the other day my cousin was on it, stalking some celebrity or something, and I couldn't understand how it works. Like on the celebrity's page there was just a list of comments about random stuff, but I couldn't work out if the celebrity wrote all the comments or if they were written by other people commenting on something she wrote. Can someone please explain to me who writes those messages, because I'm very confused.

[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.