Twitter Tip: How replies work at the start of a tweet (Video)
Have you ever tweeted using the @ reply at the beginning of a tweet but wanted it to be shared with all your followers? Starting a tweet with @username will only show up in their connect and on the feeds of those following both you and the person you reply to in the tweet. The tweet will remain in your feed but won’t show up in all your followers feeds.
Hopefully this videos explains it a little better with real examples.