The Negroni is a popular Italian cocktail. Supposedly, Count Camillo Negroni concocted it by asking the bartender, Fosco Scarselli, to strengthen his favorite cocktail, the Americano, by adding gin rather than the normal soda water. The bartender also added an orange garnish rather than the typical lemon garnish of the Americano to signify that it was a different drink.
A properly made Negroni is stirred, not shaken, and (classically) built over ice in an old fashioned or ‘rocks’ glass and garnished with a slice of orange. Outside of Italy an orange peel is often used in place of an orange slice, either is acceptable though an orange slice is more traditional.