Expandable Elements?

I have my bar positioned on the side of the screen.  I'd like to create a series of elements that expand and collapse vertically when I click on them, revlealing the contents inside.  Much like Microsoft Outlook and many other programs where you click on  Mail and your mail folders appear and you click on task and your task folders appear.  Just to clarify, I'm NOT talking about a windows explorer type of tree folder, rather the type that is used in Outlook in the image below - the ones at the bottom (mail, contacts, calendar and tasks).

http://www.winsupersite.com/images/reviews/office11_06.gif

Thank You!