What is the difference between a pellet and a BB?

A pellet is a lead (or lead-alloy) diabolo with a solid head and hollow skirt, sized in calibers like .177 and .22.

A BB is a small, hard, round steel ball, typically 4.4 to 4.5 mm. BBs are cheap and fed from magazines for fast, plinking-style shooting from smoothbore guns. Pellets are far more accurate at any range past close-up because their shape stabilizes them in flight and their soft lead engages rifling.