If I type @so_and_so on Twitter, will only this person see the message, or will everyone see it?

[A]

[Eddy]

Everyone who is fillowing you. If you only want that one person to see it click to send personal message or direct message I forgot which one it is

[Eric]

Everyone will see it. Of course your followers will see it, but someone by coincidence stumbles on your profile, then they will see it too.
If you want to send a direct message, go to their profile, next to where it says "Lists" there is another drop down menu, select "Direct Message." and send it

Note - you can only send a direct message if the user allows it. Some people have disabled direct messaging.