The plant doesn’t have roots, though its anchors (called “holdfasts”) are root-like. Most use these holdfasts to try to anchor themselves to something. In fact, most seaweed won’t survive for long just floating around.
There is red, brown and green seaweed. There are 6,000 species of red seaweed, 2,000 of brown and 1,200 of green. Green ones prefer very shallow water; the brown ones tend to prefer sub-tidal zones further out, and red ones like deeper waters. Browner ones have a stronger taste.
Though none are considered poisonous, some are considered more desirable for eating.
Seaweed is used mostly today in Asian foods.
In Western food, it had some historical importance in Irish, Icelandic and Welsh kitchens. Mostly, though, in the West, it has been harvested for fertilizer for fields, and for livestock feed.
Seaweed can be processed into agar-agar (a thickener used in Asia) or in the West, specifically Ireland, it can be processed into carrageen to be used as a thickener and a clarifier.
In Anglo-Saxon, “Fleotwyrt”, “Flotwyrt”, or “Saewar.”