Hi PinkyAnne
The # symbol is used for what is called 'trending' and does exactly what you described. it means that others can find people tweeting about the same thing. It's how the news spread so fast about Michael Jackson for example as everyone tagged their tweets #michaeljackson.
You can send anyone a message using the @ symbol but you can only send them a direct (private) message if they are following you and you are following them.
Thanks for the wishes about the swine flu. Watching JC movies all day takes my mind off of it. 1408 was great this afternoon!
Hope that helps
Lisa