Rods and cones contain photoreceptors that transfer light to vision centers in the brain. The photoreceptors work by a rather complex process: they first convert light into chemical energy, and then into electrical energy, before sending it through the optic nerve to the brain.
Rods are specialized to perceive dim light and are the first receptors to detect movement and register shapes, yet they cannot detect color. There are about 126 million rods in each eye.
Cones function best in bright daylight. They allow people to see colors and fine detail. Cones are divided into three different types, which absorb wavelengths of light in the short (blue), middle (green), and long (red) ranges. There are about 6 million cones in each eye.
Source: Tortora, Gerard J. Principles of Human Physiology, 2nd ed., pp. 280-81, 287-90.