Stacks are like groups in the taskbar but instead of open apps it's with files, right? What a ripoff.
It's basically a folder shortcut that opens up when you click it, showing you the contents at a glance.
It's actually not all that useful yet. They improved it in 10.5.2, giving you the option to use "list" view, where you can do mouse-over and open folders. The Fan and Grid views don't allow this yet... That's lame.
Stacks are kinda stupid, if you ask me.