Phaeophyta are the brown seaweeds. They are not plants, and genetic analysis shows they should be treated as a separate kingdom within the domain Eucarya. Nor are they closely related to red seaweeds, which are the kingdom of Rhodophyta. Green seaweeds are plants.
Several thousand* species are known, including kelp and wrack and other familiar large tough seaweeds washed up on beaches. Some are edible. Some are giants, and can reach 100 m long and form huge forests under the sea. The Sargasso Sea in the North Atlantic is dominated by a giant floating forest of Sargassum. But most species are anchored.
Their brown pigment is a xanthophyll called fucoxanthin. The characteristic long blade of the familiar species is called the thallus, and the fist-like lump that anchors them is called the holdfast. The trunk from which the thalli radiate is the stipe. The large bladders enable them to float but there may also be sexual sacs.
They have both sexual and asexual reproductive methods, being able to produce eggs, sperm, or spores, and some use pheromones to bring them together. They gain energy by photosynthesis, using chlorophyll a and chlorophyll c. This is another difference from plants, which use the a and b forms of chlorophyll.
Almost all are marine or coastal, with a few freshwater; none live wholly on land, though of course some of the most familiar are intertidal. These survive desiccation with the help of the gelatinous compound algin.
* Tudge says 1500, the seaweed.ucg.ie site says 3000.
Colin Tudge, The Variety of Life, Oxford, 2000