"Beef" refers to any meat that comes from a cow, whether it be a steak, a slab of ribs or hamburger.
If you are ordering steak in a restaurant or asking for it from a butcher, there's no need to specify "beef" - as a rule, "steak" implies that you are referring to beef. (Note: there are such things as pork steaks and ham steaks, but if that's the cut of meat you want, that's when you specify that it's something other than beef.) However, if you are requesting a (beef) steak, you will be asked whether you want sirloin, or New York cut, or T-bone or filet mignon.... Seems like ordering a good slab of beef is never simple. ;-):lol: