The difference a river makes. The Left Bank is planted mainly to cabernet sauvignon the Right Bank to merlot, with cabernet franc, malbec and petit verdot as additional varieties. The Bordeaux region is naturally divided by the Gironde Estuary into a Left Bank area which includes the Médoc and Graves and a Right Bank area which includes the Libournais, Bourg and Blaye.
The Médoc is itself divided into Haut-Médoc (the upstream or southern portion) and Bas-Médoc (the downstream or northern portion, often referred to simply as "Médoc").
All of these regions (except the Libournais) have their own appellation and are governed by Appellation d'origine contrôlée laws which dictate the permissible grape varieties, alcohol level, methods of pruning and picking, density of planting and appropriate yields as well as various winemaking techniques.
Bordeaux wine labels will usually include the region on the front if all the grapes have been harvested in a specific region and the wine otherwise complies with the AOC requirements.