In some respects, the name "Barleywine" tells you everything you need to know about the style; in other respects, it is incredibly deceptive. First of all, barleywines are not really wines. Wine is fermented fruit juice, usually from grapes, whereas barleywine is a potent ale produced by fermenting grain sugars.
Then why are they called "wines"? These beers' complexity and strength—two clear wine-like characteristics—were the basis for the style's moniker.
Although there are many high-alcohol beers available today, barleywines have historically been among the strongest beers available, with ABVs ranging from as low as 8% to 12% or higher. Keep in mind that before IPAs and Imperial Stouts took over the American craft beer scene, the vast majority of brews fell somewhere in the 4-5% ABV range, making the alcohol level on Barleywines much closer to what the average consumer would find in a wine instead of a beer. Barleywines are also among the few beers that age exceptionally well, much like a fine wine, thanks to their high alcohol content.
In order to increase the amount of sugars available for fermentation, brewers must add more malt, usually barley, to the beer in order to reach such a high ABV. Barleywines require additional hops to maintain balance because all that malt makes them sweeter. Because of the enormous amount of malt, hops, and alcohol, barleywines are incredibly complex beers.
American barleywine is usually hoppier, while English barleywine is mellower and more balanced. Both varieties are described by the Brewer's Association as having "flavors of bread, caramel, honey, molasses, and toffee." These dark, malty beers are enhanced by extra alcohol complexity, as the name implies.
Sierra Nevada, one of the pioneers of the American craft beer movement, is a good place for anyone interested in learning about American-style barleywines. Since 1983, the renowned California brewery's Bigfoot Barleywine has been a hipper take on the style. Meanwhile, Weyerbacher Brewing Co. in Pennsylvania offers a traditional English-style barleywine called the Blithering Idiot.