Other countries make good bubbly, but under the venerable 1919 Treaty of Versailles cannot properly call it Champagne. So it's sparkling wine in California, spumante in Italy, sekt in Germany, cava in Spain. Sugar plus yeast equals alcohol plus carbon dioxide, if you want to be a seventh-grade chemistry student about it. Top French Champagnes may be made from a single grape variety or a blend. Champagnes are also labeled according to how dry or sweet they are.
Styles vary with the wine house, too.
Jouet, Louis Roederer. Lighter, more delicate Champagnes come from Charles Heidsieck, Mumm, Taittinger. Today they compete very well with French bubblies in price and, many times, in quality. Sekt, the German name for top-quality sparkling wines, is made by a very German variation on France's methode champenoise. The bubbles come from secondary fermentation, not in bottles but in huge, pressurized fermentation tanks.