"Reverse" mask?

This might be a silly question, but I can't seem to find any tutorials on it.

I'm trying to use an all-black silhouetted figure as a mask so that you only see the layer below it within the shape of the figure; everything surrounding it should be white.

I've seen it done, and I know I've done it once before, but I've forgotten how.

Help?

Hello,
- Select the black figure, e.g with the Magic Wand tool
- In the Layers palette, choose the layer you want to mask
- Click the "mask" button at the palette bottom