Phycology or algology is the study of the algae. The word phycology is derived from the Greek word phykos, which means “seaweed.” The term algology, described in Webster's dictionary as the study of the algae, has fallen out of favor because it resembles the term algogenic which means “producing pain.” The algae are thallophytes (plants lacking roots, stems, and leaves) that have chlorophyll a as their primary photo synthetic pigment and lack a sterile covering of cells around the reproductive cells. This definition encompasses a number of plant forms that are not necessarily closely related, for example, the cyanobacteria which are closer in evolution to the bacteria than to the rest of the algae.

Algae most commonly occur in water, be it fresh water, marine, or brackish. However, they can also be found in almost every other environment on earth, from the algae growing in the snow of some American mountains to algae living in lichen associations on bare rocks, to unicellular algae in desert soils, to algae living in hot springs. In most habitats they function as the primary producers in the food chain, producing organic material from sunlight, carbon dioxide, and water. Besides forming the basic food source for these food chains, they also form the oxygen necessary for the metabolism of the consumer organisms.