Photoshop BLENDING MODES are powerful tools to speed up tonal correction and dust removal, among other things. They influence how a layer interacts with layers below it. They can be found in the upper left corner of the LAYERS PALETTE. When you create a duplicate or new layer, (some examples) you will see the word NORMAL in the top-left corner of a dialog box. If you click on the word NORMAL, it opens other choices, giving you real power at your fingertips.
Some of the Blending Modes that might interest photographers are:
MULTIPLY adds density and darkens the image.
SCREEN does the opposite of MULTIPLY
OVERLAY, SOFT LIGHT, HARD LIGHT are a mixture of MULTIPLY & SCREEN to boost image contrast.
I encourage you to experiment with these modes on duplicate or Adjustment Layers. Try adding them and adjusting their opacity for more unique combinations.