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.