What makes Twitter so popular?

[tin]

I just got a Twitter account, and I don't understand what makes it so popular. I find it very confusing, any help understanding how to use it?

1) What is twitpic? And how is it different than Twitter?

2) What does it mean on someone's Twitter page, when it has a link to someone else's page, starting with @. Like for example on Stefan Brogren's page....

@cassandrasteele You're awesome lady!!!!!

Does that mean ''cassandrasteele'' said that to him?

3) What are trending topics?

Thanks.

[I'mnotthere]

Stefan Brogren låter svenskt. Men jag förstår heller inte vad allt fuzz handlar om liksom vem behöver twitter, eller hur!