A Kir Royal (also spelled Kir Royale) is a popular Champagne cocktail usually served as an aperitif, at least in France. It is traditional made with Champagne and a little crème de cassis (blackcurrant liqueur), anywhere from 1/10 to 1/3 but most commonly 1/5 or 20%. Other liqueurs can be used, most commonly blackberry, peach and even basil liqueur as my friend Helene enjoys.
A standard Kir is made with white wine, usually whatever the bartender feels like using. Traditionally, less expensive white Burgundies were used, and the drink was originally called "blanc-cassis."
The name Kir comes from the former mayor of Dijon in Burgundy, Félix Kir, who often served it at parties after World War II. This served several purposes, including promoting two regional products, Burgundy wine and crème de cassis, producing a red drink as the Germans had stolen or drank all of the red wine, and we'll assume he liked it as well.
Originally, in the mid to late 1800s, Kir was made with red wine. A red wine Kir is commonly called a Communard or Cardinal today.
I first had a Kir, specifically a Kir Royal, within my first hour in Hong Kong in 1995. Upon arriving, I was picked up by Pierre The Champagne Poodle at the air port, whisked off to a very good Italian restaurant, and we had a Kir Royal at the bar while waiting for our table. I occasionally make then myself, usually with a decent yet inexpensive non vintage Champagne or other sparkling wine.
Some make the distinction that a Kir Royal is made with Champagne, and Kirs made with other sparkling wines are Kir Pétillants, although most do not worry about such distinctions.
You could of course use an expensive Vintage Champagne, but that really should be enjoyed by itself and unadulterated.
There are a number of variants, including using Chambord instead of crème de cassis, adding fruit such as strawberries, and even replacing the wine with beer or ale, cider, or milk!
Of course there are also many other types of Champagne cocktails as well.