St. Louis style ribs are cut from spare ribs to make a more uniform, rectangular rack.  Cutting a slab of spare ribs into St. Louis style makes the rack more uniform for easier cooking and serving.  The regular slab is more awkward in shape and may lead to irregular cooking.

St. Louis style ribs are prepared by cutting through the cartilage near the ends of the ribs.  This meat that is trimmed away is called rib tips, and can be cooked separately. More often than not there will be a loose flap of meat that runs along the slab, this is the skirt and it should also be removed.  Finally in preparing spare ribs St. Louis style is the removal of the tough membrane.  Slide a butter knife under the membrane and pry it up, at which point you can grab a hold of it and peel it off.  If it tears anywhere along the way, just repeat the process