The short answer is that Prosecco is a sparkling wine made from the grape Glera utilizing the charmat process (Metodo Italiano), with a fun and fruity flavor profile and bigger bubbles than what you’ll find in a flute of champagne.
The slightly longer answer is that, up until recently, Prosecco was just Prosecco. The name of the sparkling wine was used as a synonym for the grape (Glera), you had a reasonable expectation that it came from the Prosecco region (though not always) and there was no Denominazione di Origine Controllata e Garantita handwringing about heritage and cultural identity. That all changed around 2009 when the elevated profile of Prosecco led to stricter regulation and suddenly producers making their wines outside of DOCG specifications were left out of the brave new world of Italian bubbles. If you were a stone’s throw away from one of these newly protected areas, but not technically within its boundaries, no longer would you be able to call your sparkler Prosecco. How terribly confusing!
If you’re looking for a simpler explanation, we refer you to the first sentence in this FAQ. And the next time you have yourself a glass, consider raising a toast to all the Italian winemakers left out in the cold.