There are many different varieties of rice. They differ in amounts of nutrition and, more importantly, the type of starch. There are two types of starch in rice: amylose and amylopectin. Amylose is a long, straight starch molecule that does not gelatinize during cooking (think of making gelatin), so rice which contains more of this starch tends to cook fluffy, with separate grains. Long grain white rice has the most amylose and the least amylopectin, so it tends to be the fluffiest and least sticky. Amylose also hardens more when cool, joining tightly together and forming crystals that melt when the rice is reheated.
Rice that is high in amylose has a lower Glycemic Index number.
Amylopectin is a highly branched molecule that makes the rice sticky when it's released from the grain during cooking. Medium grain rice has more amylopectin, making it a good candidate for risottos, salads and rice pudding, which are served cold. And short grain rice has even more amylopectin and little to no amylose, so it's used most often for Asian cooking, when you want grains to be sticky so they are easier to eat with chopsticks. Then there's glutinous rice, which is very sticky when cooked, with the highest amount of amylopectin and no amylose.
White rice has the hull and bran removed, diminishing its nutritional content. But in the U.S., rice is generally enriched, with nutrients like calcium, riboflavin, iron, and niacin added. Brown rice has just the hull removed, so it has more fiber and nutrition. Converted rice is boiled or steamed before it is processed, which forces some vitamins and minerals into the kernel from the bran. Converted rice is higher in nutrients than plain white rice. And wild rice is not a grain, but a seed of a grass native to North America.
When rice cooks, the heat and liquid start permeating the surface of the rice. The starch molecules inside the rice grains start breaking down and absorb water to form a gel. The type of starch in the rice determines whether it will be fluffy or sticky.
