Answer:
Costumes are the artwork associated with a sprite. Sprites are things that move, so they can have multiple costumes. Backdrops are the artwork for the stage. The stage doesn't move, but can have multiple backdrops.
Sprites are used to make complex and interesting animations and games. A sprite is able to store images or animations with a set of properties such as position and visibility. Sprites also have properties to report interactions with other sprites or the mouse.