A cask is a vessel that some type of liquid was aged in. Bourbon, cognac, sherry, rum, ect. They then store, or age, wrapper leaf in them to flavor it. Liquor straight from the barrel is referred to as "cask strength". A prime example would be the Arturo Fuente Anejo, who's wrapper is aged in cognac barrels. Most others woul be advisable to avoid, unless you're an Acid fan. In which case you didn't deserve a serious answer.
Savvy?