You probably don't; or at least not how it's been explained to me. A keg has a single opening, into which fits a "spear" that will allow the beer out and also allows CO2 to be injected in to push the beer out. A cask has two openings and is mounted horizontally. One on the end (which becomes the bottom when mounted horizontally) into which goes the tap, and one in the side (which becomes the top when mounted horizontally) into which goes the "shrive" which has a hole banged in and plugged with a porous "soft spile" to let the air in as the beer is drawn out. The "soft spile" might be replaced with some gubbins that connects to a "breather" or other device that lets CO2 in as beer is drawn out (the beer doesn't go off so fast then). And Corny kegs … well, don't fit either description but are generally considered "kegs".
Of course you can. We are home-brewers and don't need to be tangled up in a "name" (keg or cask). I wrote a (quite lengthy) treatise on getting cask style beer out of Corny kegs. You can find it here:
https://drive.google.com/open?id=0BwzEv5tRM-5EQUhZbDNPdmV1bWc.
What "Brazilian" said above possibly refers to a recent post of mine where I had a Corny disconnect stuffed with cotton-wool (to act as a "soft spile", not a "breather"). But I was using a 2L mini-keg with Corny style disconnect posts because letting air in is of course risky (beer goes off after a few days); I wouldn't use this trick on a 19L Corny keg.
(And "BarnBrian" has just added a post referring to me … cheers Brian!).