Let's pretend I follow @HomerSimpson and @MargeSimpson and so do you.
If Homer sends an '@ message' to Marge such as
HomerSimpson: @MargeSimpson I'm hungry...
then both you and I can read it but other followers of Homer (who don't also follow Marge) won't see it unless they look at his profile page (ie the message will show up on Homer's profile page and is visible to anyone who looks at it, but it won't show up in his followers' twitter feed pages).
If Homer wants to encourage others to provide food he should write (note dot)
HomerSimpson: .@MargeSimpson I'm hungry...
and then ALL of his followers will receive it in their twitter feed.
- A profile page is what anyone will see when they look at someone's profile, eg http://twitter.com/JoBrodie
- Your twitter feed page is what YOU see when YOU log in to Twitter and look at http://twitter.com/
(It used to be possible to switch off an option so that you didn't see @ messages between Homer and Marge, then Twitter stopped this - the howls of outrage brought it back, unfortunately as something from which no-one can opt out, as far as I can tell.)
Putting D or d as the first character will send that message only to the recipient (the @ is NOT necessary) but you can only send a D message to someone if THEY are following YOU.
HomerSimpson: d MargeSimpson I'm hungry...
will go from Homer to Marge and no-one else.