Unlike blush wines that contain added sugar or grape juice concentrate, dessert wines are naturally sweet, made with grapes that "raisinate" (when grapes shrivel and sugars become concentrated) before fermentation. The flavor of dessert wines is intense, sometimes overwhelming, and reminiscent of sucking a sugar cube. Like port, the high sugar content of dessert wines, keeps them fresh up to year after opening.
There are four major types of dessert wines, each reaching its sweetness in a different fashion: late harvest, "botrytized" wines, vin santo and ice wines. Late harvest wines are currently popular with "new world" winemakers in the US, Australia and South America. Normally not as sweet as the other types, late harvest grapes stay on the vine after the normal harvest season is finished, gaining a sweeter, more concentrated flavor. The resulting wine tastes more sugary than normal table wines, but it is quite refreshing. Several grapes amenable to late harvest (and other methods) are zinfandel, sauvignon blanc, riesling, gewurztraminer and chenin blanc muscat (an aromatic grape usually reserved for dessert wines). Each grape exhibits a different personality, so approach each dessert wine variety with an open palate.
Botrytis cinerea (aka "noble rot") is a fungus that attacks grapes during harvest when the weather conditions are warm and humid. Noble rot makes the fruit shrivel and dehydrate, so when the grapes are picked and crushed, the juice is intense with sweetness. Germany and France are experts at deliberately employing this method. Sauternes, a wine region in Bordeaux, France, produces primarily dessert wines and uses semillon grapes. The Germans use riesling, and label their botrytized wine "Trockenbeerenauslese."
Italian vin santo comes from just-ripe grapes, cut off the vine and allowed to air dry to concentrate their sugars. This method creates some kick-ass alcohol content, reaching as high as 17-percent alcohol (most table wines fall between 10-15 percent). A variation of this method is also used to produce Vin de Paille, an ancient dessert wine experiencing resurgence right now in Europe.
The last method, and probably the least recognized, is Vin de Glaciere, ice wine, or Eiswein, all different names for the same method of freezing the grapes on the vine (or after picking) in order to concentrate sugars. It produces a wine that is very sweet, but balanced and not overwhelming. The grapes used are normally muscat, semillon or riesling.
White varietal dessert wines -- semillon, riesling, sauvignon blanc, vin santo -- are best enjoyed well chilled at 57-60 degrees Fahrenheit. Red varietals -- zinfandel, black muscat -- should be slightly chilled to 65-70 degrees Fahrenheit. Often they are sold in half bottles (375 milliliters), because the rich, almost syrupy contents should be sipped, not slammed.
To test out the dessert wine waters, here are some of my favorite producers: Bonny Doon's Vin de Glaciere, Quady Black Muscat, Librandi "Le Passule" Vino Passito, Inniskillin Ice Wine, Rosenblum Late Harvest Zinfandel, Santa Julia Tardio, and King Estate Late Harvest Pinot Gris.