Diatoms are unicellular algae which form distinct and beautiful cell walls from silica. They are widely distributed throughout the upper layers of the oceans of the world, and can also be found in fresh water or moist environments, such as the undersides of plants. There are over 16,000 recognized diatom species, with many more being constantly identified. Because diatoms are so plentiful, they form an important part of the pelagic food chain, serving as a food source for most of the animals in the ocean, either directly or indirectly.

Like many other algae species, diatoms photosynthesize their energy. They also have very limited mobility; some species of diatoms are capable of a slow oozing motion, but others rely on currents to carry them around the ocean. When they die, diatoms sink to the ocean bottom, contributing to the layer of sludge which makes up the sea floor. In parts of the world where oceans no longer exist, this sludge forms a fossilized layer of diatomaceous earth, a substance used in manufacturing and as a natural pesticide.

All diatoms belong to the class Bacillariophyta, although some biologists dispute over their precise classification. As a general rule, they are considered to be protists. They have a simple internal structure, and at some point in their life cycle, diatoms secrete silica to create strong cell walls. The cell walls take the form of two identical halves which interlock, much like the halves of a clam or mussel. The silica forms in a radially or bilaterally symmetric pattern, and it is often extremely complex and astounding to look at. Diatoms reproduce asexually, dividing themselves to create more diatoms.

In many cases, a diatom floats on its own through the ocean. In others, diatoms form huge colonies of individuals, linked together in a variety of ways. The unique organisms are sometimes called the jewels of the sea because of their distinctive cell walls. Many beginning biology students look at diatoms under the microscope to learn about the incredible detail which can be found in microscopic organisms. Any sample of sea water from the surface of a healthy ocean will contain a plethora of diatoms in an array of shapes to look at under a microscope.

Diatoms are related to dinoflagellates, another large class of protists which inhabits the ocean. Unlike diatoms, dinoflagellates are capable of motion, using flagellating arms to propel themselves. Some dinoflagellates also form symbiotic relationship with other organisms, another marked difference which sets them apart from diatoms. Both were identified and described by early biologists, and numerous pamphlets demonstrating the powers of the microscope used drawings of these minute organisms as illustrations.