Photoreceptors are cells which are specialised for the detection of light (for example, the rod and cone cells in the retina of the human eye, or the individual pigment cells in some invertebrates).
Photoreceptors contain deposits of pigment at one end, and an axon at the other.
The molecules of pigment actually undergo a change in shape when photons of light hit them.
This change in shape causes a change in the permeability of the cell membrane, to which the pigment is also bound, which changes the relative amounts of flow of certain ions across it.
This causes an electrical impulse (nerve impulse) to travel down the axon and, from there, onwards into the nervous system, in animals where one is present.