Use one layer as alpha map (mask) for another - how?

Hi, I do not understand how to do this in Photoshop. In After Effects, which I use a lot, this is easy. For example, I have one layer with transparent, semi transparent an opaque areas. I can then tell AE to use this layer as an alpha channel map for another layer. But how do I do this in Photoshop? I understand how to use a layer mask and draw on the same layer to create treansparent areas. But I want to use a separante layer, or even a separate image.

I use Photoshop CS 5.

Ingvar