St Louis Style Ribs are pork spareribs that are trimmed a bit more than usual. This produces a slimmer rack of ribs that fits more tidily on plates.
To make it, the brisket bone (aka breastbone) is removed, exposing cartilage where it was. The lower skirt meat is either removed, or has the outside edge trimmed.
For cooking instructions and other advice, see main entry for Pork Ribs.