Difference between outbox and sentbox? : General & Trouble Shooting

What is the difference between the outbox and sentbox in your profile?
I have two messages in my outbox that displays as 'unread', I sent them over a week ago, and I know for sure that the receivers have been online often enough to have seen it if they got it.

Is the stuff in outbox really sent? or not? I don't get it

Here is a direct quote

4.3.3: The Outbox

When you send a PM, the message goes to your Outbox. It remains in the Outbox until the recipient visits his/her Inbox, at which point it is moved from your Outbox to their Inbox. While the message remains in your Outbox you may edit or delete your post if you like. The message remains in your control until it is received.
4.3.4: The Savebox

The Savebox is provided as a place to keep important PMs. Often it is used to save valuable messages from the Inbox, and then you can use the Delete All function to discard the rest.