Agar, or agar agar, is a gelatinous substance derived from seaweed. In cooking, it is chiefly used as an ingredient in desserts throughout Japan. Commercially it is derived primarily from Gelidium amansii.
The gelling agent in agar is an unbranched polysaccharide obtained from the cell walls of some species of red algae, primarily from the genera Gelidium and Gracilaria, or seaweed (Sphaerococcus euchema). Chemically, agar is a polymer made up of galactose subunits.
Agar can come in multiple forms for culinary use. It is most commonly available in powdered or strand form. When in strands, it is often referred to as "China grass".