Skirt steak is a cut of beef steak from the plate. It is long, flat, and prized for its flavor rather than tenderness. Skirt steak has even more intense beefy flavor than flank steak. It does contain more tough muscles than flank steak, though, so should only be cooked to rare or medium rare for the most tender texture. It should also be cut against the grain when served.
Every side of the beef has 2 skirts, one inside and one outside. The outside skirt is the premium skirt. Every high end restaurant and steakhouse uses outside skirts because they are more tender. The Outer Skirt is the Ribeye of the skirts while the Inner Skirt is the sirloin.
Note: Actual product may be slightly different from the images.