PM, outbox, inbox, sent messages. What?

I’m a bit confused about the PM system.

I have some messages in the “Outbox”, and then theres other sent messages in “Sent messages”. Whats the difference?
Is the outbox sent messages or just stored for sending later, or is it PM’s that the recipient hasnt read yet? (in that case, Greenman, you got a PM).

Tried the reading faq but without any luck.

Confused and foregin.

Outbox: Messages you have sent, but have not been read by the recipient.
Sent: Messages you have sent, and have been read by the recipient.

I see. Thanks for the clarification!