The difference
Champagne is protected sparkling wine from the Champagne region of France, usually made by secondary fermentation in bottle. Prosecco is protected Italian sparkling wine that is usually fresher, fruitier, and made by the Martinotti-Charmat tank method.
How to choose
Choose Champagne or another traditional-method sparkling wine for dinner, texture, toast, and longer aging notes. Choose Prosecco for brunch, spritzes, fruit, and easygoing parties.
Why Champagne can only come from Champagne
Champagne is a protected designation tied to a specific region in north-eastern France. The name is reserved for sparkling wine grown and made there under the Champagne appellation rules. A bottle can use the same broad production idea elsewhere, but it should be called traditional-method sparkling wine, not Champagne.
Traditional-method does not mean Champagne
Many excellent sparkling wines use bottle fermentation like Champagne: Cava, Crémant, Franciacorta, Trento DOC, English sparkling wine, Cap Classique, and traditional-method bottles from California, Oregon, Australia, and New Zealand. They can be serious, dry, textured, and dinner-worthy without carrying the Champagne name.
Where Prosecco fits
Prosecco is also a protected regional name, but the style is different. Most Prosecco is made with the Martinotti-Charmat method, where the second fermentation happens in pressurized tanks. That tends to preserve fresh pear, apple, floral notes, and a softer party-friendly feel.
Label words that help
Look for "traditional method", "méthode traditionnelle", "bottle fermented", "Cava", "Crémant", "Franciacorta", "Trento DOC", or "Cap Classique" when you want the Champagne-method texture without the Champagne price.