A cask is a round wooden barrel used to hold a drink while it sits — most often wine, beer, or whisky. People also use the word for the amount inside: “a cask of wine” can mean the barrel, or that much liquid. In everyday talk most people just say barrel; cask belongs more to wine shops, whisky labels, and British beer talk. In Britain, cask ale is beer that is still alive in the cask itself, not beer that was finished in a factory tank, so the word there is a little proud, not only a container. Casks are usually oak. Though actually, the fat cellar barrel you picture is right for a lot of wine, but a cask can be smaller than that, and whisky is often put into a cask that once held something else, like old sherry. It is not a box, and it is not a metal drum.