A harmonious blend of the base beer style with characteristics from aging in contact with wood (including any alcoholic products previously in contact with the wood). The best examples will be smooth, flavorful, well-balanced and well-aged. .
The base beer style should be apparent. The wood-based character should be evident, but not so dominant as to unbalance the beer. The intensity of the wood-based flavors is based on the contact time with the wood; the age, condition, and previous usage of the barrel; and the type of wood. Any additional alcoholic products previously stored in the wood should be evident (if declared as part of the entry), but should not be so dominant as to unbalance the beer. IF THIS BEER IS BASED ON A CLASSIC STYLE (E.G., ROBUST PORTER) THEN THE SPECIFIC STYLE MUST BE SPECIFIED. CLASSIC STYLES DO NOT HAVE TO BE CITED (E.G., "PORTER" OR "BROWN ALE" IS ACCEPTABLE). THE TYPE OF WOOD MUST BE SPECIFIED IF A "VARIETAL" CHARACTER IS NOTICEABLE. (e.g., English IPA with Oak Chips, Bourbon Barrel-aged Imperial Stout, American Barleywine in an Oak Whiskey Cask). The brewer should specify any unusual ingredients in either the base style or the wood if those characteristics are noticeable. Specialty or experimental base beer styles may be specified, as long as the other specialty ingredients are identified. THIS CATEGORY SHOULD NOT BE USED FOR BASE STYLES WHERE BARREL-AGING IS A FUNDAMENTAL REQUIREMENT FOR THE STYLE (e.g., Flanders Red, Lambic, etc.).
The Lost Abbey Angel's Share Ale, J.W. Lees Harvest Ale in Port, Sherry, Lagavulin Whisky or Calvados Casks, Bush Prestige, Petrus Aged Pale, Firestone Walker Double Barrel Ale, Dominion Oak Barrel Stout, New Holland Dragons Milk, Great Divide Oak Aged Yeti Imperial Stout, Goose Island Bourbon County Stout, Le Coq Imperial Extra Double Stout, Harviestoun Old Engine Oil Special Reserve, many microbreweries have specialty beers served only on premises often directly from the cask.
You can view the complete style details on the BJCP website. The 2008 style guide is obsolete but there is a similar 2015 style that you can view below...