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Spare ribs (also called spareribs) are a variety of pork ribs, cooked and eaten in various cuisines around the world. They are the most inexpensive cut of pork ribs. They are a long cut from the lower portion of the pig, specifically the belly and breastbone, behind the shoulder, and include 11 to 13 long bones. There is a covering of meat on top of the bones as well as between them.

Spare ribs are also popular in the United States. They are generally cooked on a barbecue or on an open fire, and are served as a slab (bones and all) with a sauce. American butchers prepare two cuts:

Spare ribs are taken from the belly side of the rib cage above the sternum (breast bone) and below the back ribs which extend about 6" down from the spine. Spare ribs are flatter than the curved back ribs and contain more bone than meat. There is also quite a bit of fat which can make the ribs more tender than back ribs.

St. Louis Cut ribs are spare ribs where the sternum bone, cartilage, and the surrounding meat known as the rib tips have been removed. St. Louis Cut rib racks are almost rectangular.

Source: wikipedia.org