mailbox store and public folder store

Kidego is right on the money.  Public folders are primarily in place for collaboration.  It's easier to allow users, who delegated administration rights, to administer the public folders.   It's also easier to allow users to connect to a Public Folder or Public Folder tree than to allow users access to certain folders on a mailbox.

Public folders are also places to keep public calendars and public contact lists. They can be used for newsgroups, documents and any data that users need to share. They can keep archival copies of emails sent for quick access by multiple people. Users can create public folders and as Bob said, administer them. This is far easier than having to bug a network admin to create a new shared folder on the network give permissions to allow bill, joe, susie and the accounting dept. to access it every time they need a new central place to store things:)

Here's a really good how-to article on how public folders work and how to start taking advantage of this:
http://searchexchange.techtarget.com/originalContent/0,289142,sid43_gci1025495,00.html