Those are sweet wines, obtained by natural concentration of sugar and aromas in the grapes. There are two ways: by dessication or by noble rot.
Usually, those two phenomenons coexist at the same time. The dessication or raisining is the drying of ripe grapes, the noble rot is a fungus that accelerates the evaporation of water. Not all vintages are good producing those wines because a alternation between morning fogs and sunny afternoons is needed during several days. Late harvest and noble rot selection – in french sélection de grains nobles – are produced only with gewurztraminer, pinot gris, riesling and muscat grapes and controlled by an independant institut. This ensures a high quality product level.