Thinning out hair...

Stylists uses scissors that resemble a comb, and apply it to the underneath layers to cut off some of the strands (the percentage determined by the depth of the layer and the amount of blade in the scissors).  Does it damage your hair?  Well, it depends on how you define damage.  Does it leave fragile, broken or split ends?  Probably not -- okay in that regard.  Does it remove healthy strands of hair?  Yes, when the cut off pieces of hair regrow, the thickness returns.