Parboiling rice is a process that developed centuries ago in the villages of India. It is now widely used in Asia and the West.
Unmilled rice (still in its rough outer husk) is boiled or steam-heated for a short time. This drives the B vitamins (thiamin, riboflavin, and niacin) from the outer bran into the center of the grain, called the endosperm. The rice is cooled, dried, and milled of its husk, and then is usually milled and polished to remove the bran and germ, becoming "white" parboiled rice. Brown parboiled rice, which retains its bran coating, is also available. "Converted" rice is a term trademarked by the American company Uncle Ben's for its version of parboiled rice.
Because the nutrients have migrated to the grain's center, parboiled white rice has more B vitamins than plain white rice, which loses those vitamins when its bran is removed. However most of the rice sold in developed countries is coated with an enriching powder of B vitamins and calcium, so its nutritional value is about the same as parboiled rice, as long as you don't wash it.
Parboiled rice is slightly yellow or tan, and its grains are firmer and remain more separate after cooking. Also, the parboiling and cooling process hardens the starches in the endosperm. So, unless parboiled rice has also been precooked and dried (which turns it into instant or "Minute" rice), it takes a little longer to cook than regular rice.
Parboiled rice doesn't absorb much liquid during cooking, and because of that, it isn't appropriate for risotto, paella, or any other rice dish that cooks in a flavorful liquid.
Instead, serve parboiled rice with a highly seasoned sauce or dish, such as an Indian curry or a stew.