The loin is the back of the pig, and it runs from shoulder to leg. (If you visualize a pig and imagine that the shoulders, hams and belly have been removed, what you have left is the loin.) The loin is sold in chops and roasts, both with and without bone and the loin is cut into a number of sections. The important thing to know is that the meat is fatter at the shoulder end (also called blade end) of the loin, and leaner at the ham end (called the sirloin end). Therefore, a sirloin roast is a roast cut from the ham end and it is quite lean; a blade roast is a roast that is cut from the shoulder end of the loin and is somewhat fattier.
The leaner cuts should not be cooked past medium (and is even better served medium-rare). Loin cuts are excellent roasted, grilled, sautéed, broiled or cooked as kebabs. You may come across stew recipes that call for loin meat, for which you're better off using the less expensive shoulder. If you have the time, a home brine is an excellent way to ensure that a loin roast will not dry out.
A term you will often see is "center cut," which refers to the center part of the loin and includes the rib section (like a beef rib-eye steak). A "center-cut" roast is considered by many cooks to make one of the best pork roasts. If it is cut strictly from the rib section, it is called a rack of pork; two racks tied together form a "crown roast" of pork.
Rib chops cut from the center-cut section have the classic "chop" shape, with a round eye of meat set at the bottom of a curved bone. Other center-cut chops may include a round piece of the tenderloin (like a beef T-bone steak); these chops are superb. If you find oddly-shaped chops in the supermarket case, they are most likely end-pieces of either side of the loin.
The tenderloin is a long, lean piece of meat that runs along the length of the loin. (It is the same cut as a beef filet mignon.) It is a darker red color than the loin meat, and very tender and tasty. If the butcher leaves it attached to the loin when the loin is cut into chops, it makes up part of a T-bone steak.
Pork tenderloin is often sold whole, vacuum-packed in plastic shrink-wrap. It generally weighs about 12 to 16 ounces, although it can weigh more. Pork tenderloin is excellent roasted whole, or can be cut into medallions for sautéing and grilling, or cut into strips and stir-fried. As with the loin, care must be taken not to overcook it.