While both wines are sparkling and are apt to liven up any occasion the two sparkling beverages are different in more ways than one. First, while all Champagne is sparkling not all sparkling wine is Champagne. Champagne comes exclusively from the cool northerly French region that shares its name. Prosecco, on the other hand, comes from the warmer Veneto region located in northeastern Italy around the city of Treviso which is located about 15 miles north of Venice.
The grapes used and the way in which the two wines are made also distinguishes them from each other and impact the style of sparkling wine produced. Champagne is made from the black grapes Pinot Noir, and Meunier along with the addition of Chardonnay. The fact that they are grown in a cool region means that the wines will exhibit a taut acidity. While Prosecco is made from an indigenous Italian grape variety known as Glera, the warm growing conditions result in wines with less acidity and more stone fruit flavors.
Champagne is produced in the Traditional Method a labor-intensive process in which the still wine undergoes a second fermentation inside the bottle and thus creating fine persistent bubbles. The wines also age on the spent yeast cells for an extended period, while Prosecco is typically made by the Tank Method, although bottle fermented versions do exist. The second fermentation takes place in a pressurized tank, rather than in a bottle, producing wine with larger, coarser bubbles also there is no extended contact with the yeast cells resulting in fresh, fruity styles of sparkling wine.
The resulting style of the two wines is also quite different from one another. Champagnes range in style from the simple versions with high acid and green fruit driven flavors to complex examples that offer flavors of citrus, green apple, biscuit, and toast. Champagnes are known for their persistent fine bubbles. Prosecco, on the other hand, is a sparkling wine that offers delicate stone fruit flavors with lower acidity and complexity than Champagne. Some examples of Prosecco are fully sparkling (aka spumante) while others are just lightly fizzy (aka frizzante). Both Champagne and Prosecco come in a range of sweetness/dryness levels.
For those wanting to know more about the complexities of sparkling wines and all the different production methods, the WSET (Wine & Spirit Education Trust) offers a great curriculum to gain a deeper understanding of not only sparkling wines but the wines of the world.