Public folders are just like the calendar, contacts and email folders in your mailbox, except they're available to everyone in your workgroup.
This is one of the original features which made Microsoft Exchange the leader it is today. Check out the vintage screen shot below - guess which version of Outlook that's from.
In their wisdom, Microsoft has been "de-emphasizing" and trying to eliminate Public Folders as a feature since 2007, but the outcry from users has thus far kept them from doing so. Public Folders are still a feature of Exchange 2010, still offered by Webville's implementation of Exchange, and still popular with many Webville clients.
CalendarA public calendar folder can hold the schedule for a product rollout, trade show or any company event.
ContactsA Public folder of contacts can hold your customers, suppliers and associates. Consolidate all your phone lists into public folders on the server and give people just the permissions they need. Some of your people might need read-only access, so they can look up numbers, but not enter changes. This can solve so many problems.
Outlook contacts can contain notes, activities and links to other items. Document things here and all your team members can see what's going on.