The prized skirt steak comes from the diaphragm muscles of the animal, attached to the 6th through 12 ribs on the underside of the short plate subprimal. It's a long, thin cut of beef, and is often confused with its neighbor, the flank steak, but has an even more intense and beefy flavor.
The Tri-Tip is a triangular cut of beef that comes from the bottom portion of the sirloin. It's latin name for this muscle is the tensor fasciae latae, or TFL. In relation to the human anatomy: if you run your hand down the outside of your legs from mid-gluteal to the outside of your knee, that's your tensor fasciae latae. TMI? Maybe.