Thogru, CC BY-SA 3.0, via Wikimedia Commons — Babybackribs roh.jpg
Baby back ribs are curved rib bones taken from the top of the rib cage where it meets the spine, in the loin section. They are shorter, leaner, and more tender than spare ribs, with the meat sitting on top of and between the bones. They cook faster than spare ribs and are popular for oven roasting and grilling with dry rubs or barbecue sauce.
Mexican carnicerías use the diminutive 'costillitas' (little ribs) for baby back ribs to disambiguate from the larger 'costillas de cerdo' (cut 27 Spare ribs).