Lotus Notes has both Shared Folders & Private Folders. In Lotus Notes, a shared folder is a "normal" folder where you file things. If you set your mail preferences (ACL) to let other users read your mail, they see all the folders - the in-box and the folders you create. However, a private folder is a VERY different thing. It is a folder in a Lotus Notes application (such as mail) where only you can see it on THIS workstation. It is private to that workstation in that it is stored locally on the workstation rather than on the server within, in this case, your mail file. A private folder can only viewed on the workstation (unless roaming is enabled). This means that any other user looking at the mail file on the server or even the same user looking at it via the web-based mail cannot see the private folder.
Microsoft Exchange has a concept of a Private Folder & Public Folder. A public folder is an Outlook folder on a user's server-based mail which allows a user to put e-mails and calendar entries in them for public viewing by other Exchange/Outlook users -- That folder is public and shared with all other users on the Exchange server (domain/forest). A private folder is a "regular" folder in an e-mail account used to simply file e-mails and is not public.