First, a definition: Steaks are basically any piece of meat that falls under the category of “fast-cooking” cuts—cuts that are low enough in connective tissue that they don’t require the long cooking times that “slow-cooking” cuts require. The difference between a steak and a roast essentially comes down to size—any good roast can be cut into individual steaks (although, unfortunately, it’s not possible to put together several steaks into a large roast without the aid of transglutaminase, or, at the very least, a reliable time machine).
While cheaper cuts like sirloin, flank, and skirt, or cheffy cuts like hanger and flatiron, are becoming increasingly popular and available these days (my favorite is hanger), the kings of the steakhouse are still those cuts that come from the longissimus dorsi and the psoas major. The longissimus dorsi muscles are a pair of long, tender muscles that run down either side of the spine of the steer, outside the ribs, all the way from the neck to the hip. The tenderness of a steak is inversely related to the amount of work that a muscle does during the steer’s lifetime. So, as a relatively unused muscle, the longissimus dorsi (commonly referred to as the loin or the backstrap) is extremely tender, making it an ideal candidate for steak (and also quite expensive).