The “@”-sign is used to respond to people on twitter, and is completely user-invented:
That is, if you write “@nickhodge blah blah blah”, there is a magic URL added to nickhodge.
Now, this message is sent to the public, but only friends of @nickhodge and you see the response. That is, if person-y is your friend and not nickhodge, they will not see your @nickhodge response.
Technorati Tags: twitter