What does this symbol "#" on Twitter mean?

[mullenshane]

I need help on this you guys. Does this "#" mean to send a direct tweet to someone? Or does this symbol "@" mean to send a direct tweet? I need help on what both of those symbols mean. This would be a lot of help guys.

[Windowphobe]

@ directs a tweet to someone.

# (the hashtag) denotes a topic, especially one you'd like to see Trending. (As I type this, there's a topic called #amitheonlyone. If you'd like to add something to that topic, you'd add that hashtag to your tweet; someone searching for it will see the tweets, including yours.)