(updated on 11-23-13)
The gist is that if you send a tweet that starts with a twitter handle in the first position of your tweet (ex., @joehertvik), that message will only be sent to your followers who also follow the twitter account you reference in your tweet.
The simple way to think about this is: starting a tweet with an @ in the first position makes it a reply. It only goes to the person owning that handle and your mutual followers.
Putting the @ twitter handle anywhere else but the first position in the tweet makes it a mention and that tweet will go to all your followers.
It’s a subtle distinction in how Twitter works, but an important one. You could be intending to send a tweet out to 100s of your followers but if you put a twitter handle in the first position, you could only be reaching a subset of your followers.
Gary describes it a whole lot better than I do so go read his article. But make sure you do read it because everybody makes this mistake on twitter.
If you want to tweet out Gary’s message from the post to all of your followers along with a link to his article, send this tweet to your followers.
#protip if you start a tweet with an ‘@’, it won’t appear in your follower’s feeds unless they follow that person too http://goo.gl/1fu0Nw
The @ first position issue is also described in AllTwitter’s great article on 10 Must-Learn Lessons for Twitter Newbies. It’s a good read for basic information if you’re new to Twitter.