AOC
AOC stands for Appellation d'Origine Contrôlée, which can loosely be translated as controlled designation of origin. It is the backbone of the French wine system and a guarantee that a wine really does come from the place the label claims. An appellation lays down a long list of rules for a particular geographical area, including which grapes may be used, how much may be harvested per plot, and how the wine is grown and made. The idea is that the name of the place should vouch for a certain style and quality.
For you as a drinker, AOC is a useful guideline. When the bottle says, for example, Champagne, Alsace or Châteauneuf-du-Pape, these are precisely appellation names, and they tell you more about the wine's character than a grape name alone does. The name Champagne, for instance, is protected, so only mousserende vin from the defined area in northeastern France may call itself that, even if the same method is used elsewhere. AOC makes up more than half of French wine production.
A widespread misunderstanding is that AOC in itself is a stamp of high quality in the glass. It is first and foremost about provenance and about following the rules of the place, not a verdict on taste. Today you often see the European designation AOP, which means the same thing. As a general rule, the smaller and more specific the area an appellation covers, the more precisely it tells you about the wine.