All
diodes are sensitive to
light. Photodiodes are just
doped so that they are more sensitive. When a
photon hits the
pn junction, it causes an
electron to jump the gap. Do this with enough photons and enough photodiodes, and you can run a
pocket calculator.
When you don't want to harness the photodiode for energy, but want to use it as a light detector, you should consider using the more sensitive phototransistor instead.