A cask is a barrel-shaped container and is usually used for storing liquids - such as wines, sherry and luxury wines and fortified wines (hence Amontillado.) Casks made from wood are particularly suited to keeping wines safe and cool underground or in cool shade so that the alcohol is kept in perfect surroundings and environment for maturing. These wines are worth a lot more money the longer they mature - they then become vintage wines. The art of producing these vintages is highly skilled and ancient and the skills were often passed down from father to son in great wine-producing dynasties. This idea of heritage and ancestry is important in 'The Cask of Amontillado' by Edgar Allen Poe as it concerns the severity of making an insult against such a long-respected family.
Both Montresor and Fortunato refer to the cask as a "pipe." This word is defined in The American Heritage Dictionary (Boston: Houghton Mifflin Co., 1985) as "A wine cask having a capacity of 126 gallons." It would appear that the story is set in Venice and that the Amontillado would have come in by ship, probably from Barcelona. Montresor probably wouldn't want to drink such an enormous quantity of sherry (if it had really existed) but would have purchased it to sell in bottles at a modest profit. Early in the story Montresor seems to reveal that both he and Fortunato deal in expensive luxury items such as art work, jewels, and antiques. Fortunato is no doubt interested in the Amontillado sherry because he thinks he can make a profit if he acts quickly. Montresor is a poor man and could only (according to him) buy one cask; but Fortunato is rich and could buy a whole shipload--if it turns out to be real Amontillado. Since he has heard nothing about it, he guesses that the cargo has just arrived. One advantage in dealing in wooden casks of fine wine is that it would only improve with age. He could sell it off in small quantities at his leisure. Of course, the cask of Amontillado does not really exist. Montresor invented it to lure Fortunato to his death in the catacombs.