Barley wine is a strong ale that got its start in England in the 1800s. The term ‘malt wine’ was used in the 18th century, but barley wine was used in the 19th century to market to common folk as an alternative to wine made with grapes. They were made using the first, high gravity runnings of the brewer’s mash. At the time, it was common to brew multiple beers (2 or 3) from the same mash.
Nowadays, barley wines are brewed specifically. These ales are complex and their flavor profile can change with conditioning time.
This style has some loose guidelines when it comes to color and, in some instances, the amount of alcohol in the brew. I think that they should be strong and over 8% ABV.
With this looseness out of the way, a brewer can focus on the concrete guidelines around the flavor and the mouthfeel.
Barley wines should be malty. It should be apparent in the aroma. The flavor profile should have many layers to it. It should hit upon all the malty descriptors from nutty to bready to caramel to roasty to toasty to molasses. From what I have read, most of these flavors should come from just pale malt…but using long boiling times to caramelize the sugars (of course specialty malts can help, especially for extract brewers).
The mouthfeel should have a lot of body. It should have a chewy feel to it.
BCJP breaks this style into two types: English and American. The American guideline allows for more hop flavors…naturally.
From what I read, this seems to be a tough beer to brew. The starting gravities tend to be very high. I think one would need to have their fermentation process down. They would need to have experience making a starter, using yeast nutrients, and controlling their fermentation temps. The use of a secondary fermenter may also be necessary.
I think the best part of the barley wine style is its changing flavor profile with age. This style would be good to brew and age…and taste at different points in its lifespan.