The bubbles in sparkling wine are carbon dioxide captured from a second fermentation. How that fermentation happens decides style, quality and price.
Traditional method
The second fermentation takes place in the bottle, as in Champagne, Cava and Franciacorta. The wine then ages on the lees, giving fine, persistent bubbles and notes of bread, brioche and toast. It's the most labour-intensive method with the highest quality potential.
Tank method (Charmat)
The second fermentation happens in a large pressure tank, as for Prosecco. The method preserves fresh, primary fruit and gives a lighter, fruitier and usually cheaper wine.
Sweetness
Sparkling wine is graded by sweetness: from brut nature (bone dry) through brut (dry, most common) to demi-sec (medium sweet).
Serving
Serve well chilled, around 6–8 °C, in a tall glass that preserves the bubbles. Skip the wide coupes — they let the fizz escape too fast.