Loading a layer as a selection, adding to, or Intersecting the selection
To load a layer as a selection, press-and-hold the Command (PC: Ctrl) key and click on the layer's thumbnail. If you Command-Shift-click (PC: Ctrl-Shift-click), it will add that layer to the already existing selection. If you Command-Option-click (PC: Ctrl-Alt-click), it will subtract from the selection. If you Command-Option-Shift-click (PC: Ctrl-Alt-Shift-click), it will intersect the selection.