The sprites of a game are the players character and any NPCs (Non-Playable Character) you may have in your game.
Sprites are made up of multiple still images (or frames), that when put together sequentially create an animated sprite.
A sprite sheet is used to layout all the still images of the sprites to keep this organised and is used to speed up the process of creating the animated sprite by putting all still images on the one sheet instead of many individual ones.
2D Sprites – Game Art 2D
Pixel Sprites – Edge Magazine (THE ART OF THE PIXEL)