A stack is a Dock item that gives you fast access to a folder. When you click a Stack, the files within spring from the Dock in a fan or a grid, depending on the number of items (or the preference you set). Snow Leopard starts you off with three default Stacks: one for Applications, one for downloads, and the other for documents.
here is a application stack,
If you need the same feature in windows 7, there is one free add on called 7Stack. you can get some effect like this
Steps, Download the 7Stack
create one stack, basically Chose one folder, and assign the options, then click generate , it will create one shortcut. you can drag and drop the shortcut to the taskbar.