What's the difference between curved-jaw and straight-jaw locking pliers?

Curved-jaw pliers grip round and irregular shapes (bolts, rounded fasteners, pipe) with four points of contact, which is why most reviewers recommend them as the default first pair. Straight-jaw versions excel at flat stock and sheet metal where you want even pressure across the jaw face. Most multi-piece sets reviewed here lean curved-jaw plus a long-nose for tight spots.