Jealous of wine makers, brewers started to age their beers in barrels. Usually stouts/porters or wild ales styles are aged. Expect depth, more alcohol and often additional ingredients (chocolate, coffee, fruits). Barrels used are usually ex-Bourbon, Ex-Scotch but sometimes ex-Rum or even ex-Tequila.
Founders KBS, Heart Of Darkness Ghost and Anderson Valley Huge Arker Stout are typical great examples.