In the production of Champagne and the wines made in the same style, there are many complexities involved. However, there is one essential process that separates the traditional method - a secondary fermentation within the bottle the wine is sold in.
Once the base wines are blended (assemblage to the French), the second fermentation begins. This second fermentation is the most important step in the whole process and is, in fact, the only way to produce a fully sparkling wine. At this point, a mixture of still wine, sugar, yeast, and a clarifying agent called the liqueur de tirage, or bottling liquor, is added to the final cuvée. The wine is then bottled with a temporary, beer bottle-like crown cap. The added sugar and yeast react with one another, creating a second fermentation inside the bottle. Fermentation converts the sugar into alcohol and carbon dioxide which now gets trapped inside the bottle, infusing the wine with bubbles.