Enriched flour is flower that has certain vitamin supplements added to it. The specific nutrients are as follows (from WiseGeek article): "Each pound (0.45 kg) of flour must contain the following nutrients in order to be considered enriched according to the United States Food and Drug Administration (FDA) guidelines: 2.9 milligrams of thiamine, 24 milligrams of niacin, 0.7 milligrams of folic acid, 1.8 milligrams of riboflavin, and 20 milligrams of iron. Except for iron, all of these nutrients are B vitamins. "

Nutrients are added to white flours because it lacks many nutrients (as compared to darker flours). Since this was promoted widely in World War II, it has come to be considered in part a component of "war readiness" by making sure the troops and supporters of the military were well-fed and nourished. It doesn't mean that now, but that is part of the historical context for enriching (adding nutrients) to flour.

There is a myth/suggestion/notion that enriched flour is bad for you. This is not specifically true, unless you have allergies of some sort. Instead, white flour is so heavily processed that it contains very little fiber or nutrients on its own. Enriching adds some back in, but eating products made from darker flours avoids the whole cycle of removal/re-addition of nutrients, and so is both less energy intensive in production, and perhaps healthier to begin with. The blame placed on enriched flour is because it is so widely used that sometimes people consume little else as a flour product, and like anything, even water, in sufficient quantity it can have negative impact on you. If you have the choice you're probably better off using non-enriched darker flour, but that's up to you, your doctor/nutritionist and your wallet.

FDA standards are that there have to be certain amounts of Niacin, Riboflavin and Thiamine to be called 'enriched'. there also may be calcium added up to a certain quantity as well.

enriched flour is regular flour with added nutrients like thiamin, riboflavin, niacin, folic acid and iron.