Rods and cones are not distributed evenly; most cones are in the fovea and none are in the blind spot.  Rods are spread out more evenly than the cones, however there are no rods in the fovea or the blind spot.

When light enters the eye it is absorbed in the outer segment by light sensitive photochemicals (which are manufactured in the center of the cell).  A complex chemical process then generates an electrical signal that moves through the cell to the synaptic ending.  After that another chemical (a neurotransmitter) is released by the cell and passes across a gap (the synapse), to the nerve endings in the plexiform layer.

Each rod contains a stack of disks which contain photochemicals.  Some of the photochemical is used up when it absorbs light.  The central part of the cell produces photochemicals to replenish the outer segment.  The new disks produced continuously push out the outer disks, which break off the end of the cell and are broken down in the choroid.

Photochemicals in the rods are made up of a derivative of vitamin A as well as a protein, opsin.  The purple color gives the chemical its name, rhodopsin.

Rhodon - (Greek) rose
Opsis - (Greek) sight

When light in the visible spectrum strikes a molecule of rhodopsin, it causes the electrons in the molecule to resonate which in turn changes the molecule.  In that process, an electrical signal is generated and it becomes the signal which is to be transmitted to the rod synapse.  The new molecule than decays to its original components (vitamin A derivative and opsin).  During the breakdown the molecule becomes lighter in color and no longer absorbs visible light (it turns white), a process usually called bleaching.  At the end of the breakdown enzymes rebuild rhodopsin, however that cycle does not repeat forever, so the disks must be replenished.

The cone cells work in similar ways but they are less sensitive to light in general as well as light coming in at stray angles.  This way they can be used to produce sharper images.