Sloes are tiny berries that grow wild in hedgerows around England. Unpleasantly astringent on their own, they are popular in sweetened jams and preserves, and they develop a rich, tart flavor when infused in gin. Naturally, enterprising distillers have been incorporating them into liquor since the 17th century. The spirit takes on the essence and bright color of the berries, and then sugar is typically added to counter the fruit's tartness. What's left is not technically gin, but actually a gin-based liqueur.
The British traditionally used sloe gin in wintry drinks, but it's become most famous for its turn in America's refreshing Sloe Gin Fizz, where it's paired with club soda, citrus and simple syrup. This synergistic combo yields a cocktail that is sharp and quaffable, with sloe gin's characteristic reddish purple hue.
The Sloe Gin Fizz is certainly the most famous and craft-focused cocktail to utilize the colorful spirit, but sloe gin can also be found in '80s-era favorites like the Alabama Slammer. However, while that college staple may feature the liqueur, it doesn't highlight it front-and-center like the fizz.