Seaweeds can be classified into three broad groups based on colour: brown, red and green. Botanists refer to these broad groups as Phaeophyceae, Rhodophyceae and Chlorophyceae respectively. Brown seaweeds are usually large and range from the giant kelp that is often 20 metres long, to thick, leather-like seaweeds from two-four metres long, to smaller species from 30-60 cm long. Red seaweeds are usually smaller, generally ranging from a few centimetres to about a metre in length; however red seaweeds are not always red, they are sometimes purple, even brownish red, but they are still classified by botanists as Rhodophyceae because of other characteristics. Green seaweeds are also small, a similar range in size to the red seaweeds.

Seaweeds are also called macroalgae. This distinguishes them from microalgae which are microscopic in size, often unicellular and are best known by the blue-green algae that sometimes bloom and contaminate rivers and streams.

Naturally growing seaweeds are sometimes referred to as wild seaweeds, in contrast to seaweeds that are cultivated or farmed.