PHOTOSHOP - Mask Layers

Layer masks are really handy for when you want to hide or reveal parts of a layer (adjustments layers, or image layers for example).

SELECTIVE COLOUR EXAMPLE
Selective colour is when most of the image is in black and white and onnly a small section is coloured. It is very simple to do with Layer Masks.

Open your image. Add a black and white adjustment layer.

With that new adjustment layer still selected in your layers window, add a vector mask (third button from the left along the bottom of the layer window).

In Photoshop 6, once you add a vector mask the foreground and background colour swatches in the tool box automatically change to white and black respectively.

white = reveals
black = hides

Switch around the foreground and background colours by clicking on the curved arrow just about the swatches. Black should now be the foreground colour (and therefore selected).
Choose the paint brush from your tool box.
Start painting on your image where you want hide the black and white layer and show colour.