The term “algae” has traditionally been applied to all photosynthetic eukaryotes except the land plants, and has sometimes included the cyanobacteria, a group of prokaryotes. The algae were seen as differing from the land plants by their lack of roots, leaves, and stems, their lack of conducting tissues like xylem and phloem (except in some brown algae), and their lack of some of the complex reproductive structures seen in land plants. The algae, however, are not a monophyletic group, and represent many different clades within the eukaryotes. While some groups of algae are related to each other (like the brown algae, golden algae, diatoms, golden algae, and yellow-green algae), others are not phylogenetically related to each other in any way except that they are eukaryotes. However, the eukaryotic algae are ecologically important because, along with cyanobacteria, they are often the most important photosynthetic organisms in aquatic environments.

Members algae are found in diverse habitats: from ice-covered lakes in the Antarctic to hot springs in Yellowstone Park; from 300 m. deep in tropical oceans to 3300 m. elevation on top of the snow pack; from small algae that are symbiotic with corals to giant kelps, the longest "plants" in the world; and from parasitic algae found on the human skin to seaweeds which produce useful products used in skin creams and cosmetics. In the ocean and in many lakes and streams, the algae supply the primary photosynthesizers, and are therefore the main source of organic matter for heterotrophic organism from bacteria to whales. In the oceans and in lakes and streams, algae occur in two ecological categories: phytoplankton, microscopic algae that are suspended in the water column and seaweeds, which are attached to the bottom and are generally found in coastal areas.