Barley wine is not actually a wine but, being made from malt and hops, a beer. As a general rule barley wine is similar to strong ale, only stronger (between 10 and 13 per cent ABV). Barley wine is a beer of English origin, although it is brewed comercially in America too nowadays. Barley wine is bottle conditioned (or cask conditioned) and thus can be aged for a while.
Due to its high alcohol content it should be matured for at least 6 months and some people will cellar it for 5 years.
Barley wine is made using a top fermenting ale yeast. It has little perceived bitterness, even with a reasonable amount of hopping due to the large amount of malt. Old Nick is one of the easier barley wines to find (YMMV natch).