Like France, like Italy, German viticulture is divided into a number of regions, the most important being the Mosel, the MittelRhein, the Rheingau, the Nahe, the Rheinhessen, the Pfalz and Baden.
In addition, the Mosel is really three regions in one: the Mosel itself, the Saar, and the Ruwer.
Each region has its own rules and traditions, most of which are quite subtle for the general reader and wine drinker. Suffice it to say that the northern regions, Mosel, MittelRhein, Nahe, and Rheingau, tend to produce lighter, more delicate wine. And the southern regions, Rheinhessen, Pfalz and Baden tend to produce fuller bodied wines.
Riesling is the main grape in the north; it is joined by Pinot Blanc (Weissburgunder), Pinot Gris (Grauburgunder), Pinot Noir (Spätburgunder), and a host of other fascinating, mostly white grapes.