Derived from a type of red seaweed, agar-agar was once produced through a complex process of sun-drying and snow-chilling, but these days it is freeze-dried in factories and sold in sticks or powdered.
To use agar-agar in stick form, first break it into pieces and soak the pieces in water for at least 20 minutes. Once they have softened, wring out the water and add the pieces to the liquid you are using. Powdered agar-agar can just be added to the liquid, which is simmered over medium heat until the agar-agar dissolves. Don't stir until the liquid has started simmering and don't bring the liquid to a furious boil or some of the agar-agar will be deposited on the side of the pan.
Agar-agar begins to set at around 102°, so you need to work quickly once you take it off the stove. And although it will gel at room temperature, desserts made with agar-agar taste better with a couple hours of chilling. The texture is slightly different from gelatin: pleasantly firm, with less rubbery jiggle. We especially like it when used to make coffee jelly and other Asian-style desserts.