Grouping Layers  allows you to use Shapes and Text to mask photographs and pixel information. To mask a photograph with some text simply place the layer that contains the photograph one level above the text layer and select Layer /Group with Previous from the menu.  If you take a look at the Layers Palette you’ll see that the Layer with the photograph is now indented slightly with a downward pointing arrow.

A Grouped Layer

The Original Text

The Text masking the Photograph