there are many
kombu = kelp
nori = used in rolls
wakame = used in soups
just to name a few
really need the dish to be more accurate
Carrageenan is derived from red seaweed. It is used as a thickener in cooking.
Nori or Kombu
Agar is derived from seaweed and is often used in Japanese desserts
Nori is one kind of edible seaweed used in Japanese cookery.
Nori is an edible seaweed used in the preparation of sushi or onigiri. The seaweed is essentially a wrapper for these dishes.
Hippophae rhamnoides
The Māori name for seaweed is "karengo." It is traditionally harvested and used in various ways, including as food and in cultural practices. Different types of seaweed may have specific names, but "karengo" generally refers to edible seaweed in Māori culture.
Detailed list: http://en.wikipedia.org/wiki/Seaweed#Uses
Agar is made from a type of seaweed called red algae. The seaweed is boiled and processed to extract the agar, which is a gelatinous substance used as a thickening agent in cooking and as a medium for culturing microorganisms in laboratories.
Euchema cottonii
Agar powder is a gelatinous substance derived from seaweed, often used as a vegetarian alternative to gelatin in cooking and food preparation. It is commonly used as a thickening agent in desserts, soups, and sauces, and can also be used to create solidified food items.
Seaweed is used as food for animals and humans. (suprisingly!)