This is a pretty general question, and I'm sorry I don't have a screenshot for you.
First, some specs: I am running OSX Lion on a MacMini with a Magic Mouse.
I had a screen appear and I'm not sure what it was or how it appeared. All my windows faded out and there was a list of files (and folders perhaps) listed above the dock station. This was not mission control or launchpad, I know that much. It also wasn't whatever the screen is that I get when I two-finger swipe to the right (with the calculator, clock, weather and other widgets).
If it helps at all, when it happend I was attempting to open my DropBox folder. I am often switching between a Windows PC and my Mac. I've been on my Windows computer lately and am used to opening DropBox by double-clicking the icon in the task bar. It was somewhere around that time this screen appeared. It disappeared when I clicked anywhere outside the file icons - which I unfortunately did pretty quickly.
If there is any other information I can give to possibly identify this screen, just let me know.