Green and brown algae are two groups that together make up most of the algae in the world, though they are quite different. Along with red algae, both brown and green varieties are sometimes referred to using the colloquial term "seaweeds." Though both are eukaryotic (complex-celled) multicellular organisms, they belong to different kingdoms, with green algae belonging to Plantae and brown algae to Chromalveolata. Plantae and Chromalveolata are two of the six major eukaryote divisions, the others being Fungi, Animalia, Amoebozoa, Rhizaria, and Excavata. Both groups are mostly marine, but green is better adapted to fresh water than brown.
Brown algae is most familiar to people as kelp, marine algae with a very high growth rate, and Sargassum, a surface-floating variety found on the Sargasso Sea that provides a unique habitat for eels and other animals. Though kelp and Sargassum are the best known varieties, there are over 1,500 species in total, and they are especially common in the colder Northern Hemisphere. Brown algae can often be found along rocky shores. Along with their mostly unicellular relatives in the phylum Heterokontophyta, they are autotrophs (photosynthesizing organisms) with chloroplasts covered in four membranes. This algae uses a pigment called fucoxanthin to absorb sunlight, giving it a brownish-green color. Cells within it often have holes used to share nutrients and free carbon.
Green algae is somewhat more common than brown from the perspective of humans, as it grows more frequently in and near lakes and rivers, which people tend to see more often than the open sea. It is famous for being the most primitive group in the kingdom Plantae, and the form of life from which land plants (embryophytes) evolved approximately 500 million years ago, during the Ordovician period. There are about 6,000 known species of green algae, most of them unicellular, though the most visible species live in colonies structured in long chains or filaments. Only in the order Charales — the stoneworts, a type of pond weeds most closely related to land plants — does true tissue differentiation occur.
Both types of algae are extremely important as producers in aquatic ecosystems, and the diet of many fish, especially juveniles, is mostly or exclusively made up of them. Some fish are even specially adapted to cleaning algae off of other fish. Alongside corals, kelp forests create one of the most species-rich and complex aquatic ecosystems on the planet, home to tens or thousands of marine species.