How do you send direct messages on Twitter?

[Bob Vila]

Just put a d in front of their user name instead of an @. For instance if a person's username is abc123, type d abc123 then your message. Note that is d then a space then the username. You can do this from the twitter homepage, an external twitter app, or even from IM on your phone if you have set up your phone number on your twitter account.

@abc123 will allow that person to find mentions for that username, even if they don't follow you. But those messages will be visible to everyone.