How @replies work on Twitter (and how they might)

@replies are used for Twitter conversations. You can send an @reply (@twitterusername) to direct a message or question etc. to anyone (even if they are not following you)

Remember to click on your @reply tab to see tweets directed to you.

@Replies To You

If someone @replies you—note, this means they start a tweet with @yourusername—you will see that in your main timeline if you follow the person. But you’ll see it in your replies tab, whether you follow them or not (unless you’ve blocked them)

via Twitter Blog: How @replies work on Twitter (and how they might).