Twitter 101 - Replying and Tagging Other Users
Quick Twitter Tip: The @ reply function can be tricky to those still new to Twitter.
At its most straightforward, you use @username to include another Twitter user in your tweet. When you reply, the tweet begins with their username.
When this happens, only the users who follow both you and the person to which you've replied can see that tweet. This means that if you were intending for everybody who follows you to see it, well, they won't be able to.
To ensure everybody sees it, you can either put their username in as part of your response, or put a symbol in front of the @. E.g:
.@username this is my response to what you just said
Now, everybody who reads your tweets will see what you said without having to have followed the other person first!