Red algae consist of unicellular forms or a variety of multicellular forms that grow to three or more feet (1 m) in length. All red algae store energy in a starch molecule called floridean, made up of glucose units in α-1, 4 and α-1, 6 linkages. The same α-1, 4 linkages in nature connect glucose units in vegetable starch and animal glycogen. Bacteria use α-1, 6 linkages when storing their sugars.

Red algae contain the red pigment phycoerythrin and the blue pigment phycocyanin, and each of these pigments gives red algae the unique ability to live at depths where other algae cannot. Light from the violet to blue section of the spectrum is the only light that penetrates ocean waters to depths of more than 650 feet (200 m) or more than 800 feet (250 m) in clear waters. At these depths red algae's pigments absorb light in the violet-blue spectrum and transfer the light energy to chlorophyll a in the cell's chloroplast. The algae's distinctive red color turns to blue-brown or brown-green when the pigment phycoerythrin breaks down during exposure to bright light. Divers would expect to find the reddest algae in deepwater habitats and less-red algae nearer the surface.

Red algae supply agar, the gelatinous polymer used for preparing solid and semisolid growth media used in microbiology laboratories. In living algae, the agar combines with three additional polymers to form a matrix that strengthens the cells. Outside laboratories, beachgoers sometimes find slippery and rubbery red seaweeds. These characteristics are produced by long polymer compounds that make red seaweeds indestructible in ocean currents. In addition to red seaweeds of the phylum Rhodophyta, the red algae group also contains the green seaweeds Chlorophyta and the brown seaweeds Phaeophyta. The following lists the familiar types of seaweeds and their characteristics.