Loin is the whole muscle group...tenderloin is the prime cut, like the tender loin side of a beef T-Bone vs the strip side (New York Steak). If you have a center loin chop, it is basically the same as a beef T-Bone steak. Tenderloin, as a whole muscle, is a much smaller piece compared to the cross-section of the loin, but even more lean, and more tender...suitable for cooking to lower finished temps. Tenderloin is more expensive than loin.
Eric