The main difference between fluorescence and absorbance is that absorption is the process that eats up a photon and puts a molecule or atom into an excited state. Fluorescence is the process that first devours a photon and places a molecule or atom in an excited state, then emits low energy photons that transport the molecule or atom to the ground state.
The amount of light absorbed by a substance is measured in absorption spectrometer while fluorescence spectrometry measures the intensity or amount of light emitted by a molecule.