If you post one, everyone that follows you will see it. I also think it goes out in a public timeline.
You can post a message directed at one person by putting @username at the beginning, but everyone will still see it.
If you want to say something privately to someone you have to be following them and they have to be following you. Then you use the DM thing that you practiced on me that time.
: If I post a tweet, will ALL the people I follow see the post?
: I want to post to just one person on my list.
: I don't think I understand this yet!