The photic zone refers to the portion of the water column in a lake, sea, or ocean that receives light, while the aphotic zone is the part of the water that doesn't. Because light is required to photosynthesize, a body of water's primary productivity (the amount of biomass generated directly by the sun's energy) is directly proportional to the size of the aphotic zone. Formally, this zone begins where less than 1% of light penetrates.
In the ocean especially, the portion of the water that doesn't get any light is a very deep area. It extends from 3,000 - 15,000 feet (0.9 - 4.6 km) in depth. A variety of unusual animals live here, including the giant squid, vampire squid, gulper eel, angler fish, and many others. Quite a few animals in this zone are bioluminescent, which means that they are capable of generating their own light.
Where the aphotic zone begins depends on the number of particles suspended in the water, a quality called turbidity. It ranges significantly based on the season, whether it has rained recently, the type and location of the body of water, and other factors. Turbidity can be roughly measured by a simple device called a Secchi disk, which consists of a disk covered in an alternating black and white pattern. The disk is put on a rod and lowered into the water. The point at which the pattern becomes impossible to see tells the viewer roughly how turbid the water is. For a more accurate measurement, a device called a nephelometer is used.
In the world's oceans, the aphotic zones are relatively deserted in comparison to the photic zones. As a comparison, a person can imagine taking a gigantic sunshade and using it to enclose everything in a patch of forest — eventually, most forms of life in the shaded part of the forest would die. In the oceans, however, detritus raining from above provides a source of food, so some life can survive. Other animals spend time in this zone, but ascend to the photic zone to feed.