Same (assuming both are either Chevy or GMC): Body, fenders, grille, sometimes engine and trans, some electrical, most interior parts.
Different (same assumption as above): Springs and all spring hardware, axles, brakes, wheel parts, driveshaft, bumper hardware.
This is just a partial list off the top if my head, I'm sure others will add to it. There are significant differences between Chevy and GMC of the same year. They used different engines/cooling systems/grounding; the bodies had minor differences as well (e.g. hood shape, dash layout).
