Granulated white sugar is essentially pure sucrose, whether derived from tropical sugar cane, or temperate sugar beets. White sugars vary mostly in the size of their crystals. The degree of processing by the manufacturer may also account for different degrees of whiteness.
Brown sugars are of two basic types: sticky and free-flowing. Both are produced by adding a suitable type of syrup (these days it's usually molasses) to purified or refined sugar. The color and texture of the final product are determined by the ratio of sugar to syrup in the mix, as well the the original colors of the constituent ingredients.
For additional culinary information about sugar, we headed to the Epicurious Food Dictionary. There we found descriptions of raw sugars, tips for storage, and a list of sugar substitutions for cooks and bakers.
One more noteworthy difference: a cup of brown sugar has slightly more calories than white, but it "also contains 187 milligrams of calcium, 56 mg of phosphorous, 4.8 mg of iron, 757 mg of potassium and 97 mg of sodium, compared to only scant traces of those nutrients found in granulated sugar." Sweet!
