The active ingredient in most nail polish removers is Acetone. This is what gives it that characteristic harsh / pungent smell.

Lacquer Thinner is a blend of various organic solvents -- usually Toluene and Xylene, but can also contain MEK (Methyl Ethyl Ketone) or Acetone.

Lacquer Thinner will have a more "aromatic" (think petroleum distillates) odor than straight Acetone.