Agar is a seaweed-based (and is, therefore, suitable for vegetarians) thickening agent that comes in either powder form or translucent strands. Found in most Asian grocery stores, it is suggested that agar provides a crunchier (or stiffer) texture to the food than Gelatin. Normally come in the form of powder, Agar should be first dissolved in cold liquid before being brought to a boil for the setting to occur. Agar is widely used in making jelly and many Asian desserts.
See Gelatin for more comparison between these two thickening agents.