whats the # thing on twitter for?

[Happiness]

It's a hashtag. A hash tag is simply a way for people to search for tweets that have a common topic. For example, if you search on #LOST (or #Lost or #lost, because it's not case-sensitive), you'll get a list of tweets related to the TV show. What you won't get are tweets that say "I lost my wallet yesterday" because "lost" isn't preceded by the hash tag.

The good thing about the hash tag is that if someone wrote a tweet without putting the word LOST in the main message, it will still show up in your search because of the tag. Eg. "Who is hotter? Jack, Sawyer, or Desmond? #lost"