In the culinary arts, the term cutlet is used to refer to a thin cut of meat usually taken from the leg or rib section of veal, pork or lamb. Chicken cutlets can be made from a thinly sliced and pounded chicken breast. Similarly, a turkey cutlet can be made from a thin slice of a turkey breast.
Cutlets are typically pan-fried, usually after dredging in flour and/or coating them in breadcrumbs. The classic veal piccata recipe and chicken piccata recipe are made from veal and chicken cutlets, respectively.
Beef cube steak, which is used for making chicken-fried steak or Swiss steak, is sometimes referred to as a beef cutlet. Cube steak is usually a thin cut of meat from the beef round primal which is run through a mechanical tenderizer, which produces the characteristic cube-shaped marks. This is done because beef round is a relatively tough cut of meat because it's a muscle that gets a lot of work. The cuber helps break up the connective tissue that causes toughness. This can also be done using a tenderizing mallet.
A Croquette is NOT a Cutlet
For some reason, the word cutlet is sometimes used to refer to something that is actually a croquette: in other words, a mixture of mashed potatoes or rice mixed up with other chopped veggies, fish, poultry or meat, which are formed into shapes, then dredged and fried as described above. Thus, what they have in common is the dredging and frying. However, what makes a cutlet a cutlet is that it's made from a thin slice of meat, not the fact that it's dredged and fried.
Pork is a convenient meat for making cutlets because the shape of the loin lends itself to producing thin cuts of uniform size and shape. You'll often see this exact thing described as schnitzel. The traditional Austrian weiner schnitzel is made from veal, in which case it is usually made from center cuts from the leg muscle. While the meat itself tender, it must still be carefully fabricated to remove any connective tissue that could be chewy after it's cooked.
Pounding the cutlet flattens it, which in turn makes it cook more quickly. The advantage of this is that since the meat is usually from a tougher section of the animal, you don't want to cook it any longer than you have to.
Conversely, cutlets made from chicken and turkey come from the breast, which is already tender. But the advantage of slicing it thinly and then pounding it is that it cooks through quickly, which is important since poultry needs to be cooked well-done.
Here's an easy pork cutlet recipe.