Outbox
Transfers a message from your Sent-folder to the Outbox if you want to take another attempt to send it.

Sent
Transfers a message from your Outbox to the Sent-folder.

New

Archive
See "Folder | New - Archive"

Queue
See "Folder | New - Queue"

[Folder Name]
Transfers the current article (when in the article viewer) resp. the highlighted messages (when in the articles list) to the specified folder to store it there for later reference. You can trace that message by using Special | Search in Folders