Sheets of glass
Think of each layer as a sheet of glass stacked over the photograph. Paint on the top sheet and you have not touched the photo underneath — pull the sheet away and the original is untouched. Everything else about layers follows from this.
The stack is the order of operations
Photoshop composites from the bottom up. A layer only affects what is below it, which is why an adjustment layer dragged to the bottom of the stack appears to do nothing. When an edit is not showing up, check its position before you check its settings.
The layer types you will actually use
- Pixel layers — actual image data. Photos, painted retouching, pasted elements.
- Adjustment layers — a live, re-editable colour or tonal change (Curves, Hue/Saturation, Levels). They carry a mask by default.
- Smart Objects — a container holding the original data, so filters and transforms stay re-editable.
- Groups — folders. A mask on a group applies to everything inside it, which is the tidiest way to limit a whole set of edits to one area.
Opacity and Fill are not the same
Opacity fades the layer and its layer styles. Fill fades the pixels but leaves styles like Stroke and Drop Shadow at full strength. For most work Opacity is what you want, but Fill is the trick behind a lot of text effects.
Clipping is the sharpest tool here
Alt-click (Option-click on Mac) the line between two layers and the upper one clips to the lower — it now only shows where the layer beneath has pixels. This is how you confine a Curves adjustment to a single object without drawing any mask at all. More in layer masks explained.
Name things
A file with forty layers called "Layer 42 copy 3" is a file you will not want to reopen. Name layers as you make them and group them by purpose — retouch, tone, colour, output. It costs seconds and saves hours, especially once you start compositing.