Foods that are enriched or fortified have one or more nutrients such as vitamins or minerals added to them.
Enriching a food means the manufacturer added nutrients to replace vitamins or minerals that were lost. For example, refining wheat to make white flour removes a lot of the B-complex vitamins found in the outer portion of the grain, so B-complex vitamins are added back into the flour.
Fortified foods have extra nutrients added to them too. But in this case those nutrients aren't normally found in the unprocessed state. Fortified foods are more common than enriched foods and include calcium fortified orange juice, iodine fortified salt and vitamin D fortified milk.
Academy of Nutrition and Dietetics. "Enriched, Fortified: What's the Difference?" Accessed February 12, 2008. http://www.eatright.org/cps/rde/xchg/SID-5303FFEA-D13B3A75/ada/hs.xsl/home_8388_ENU_HTML.htm.