Pellets, despite their name immediately bringing to mind the image of BB's, are more like bullets. Typically they're made of a soft, semi-malleable metal that will flatten on impact to avoid injury and keep you from reusing them over and over. They vary in shape from completely flat tipped, to hollow point .22 Calibre pellets about the size of your pinky fingernail.
Pellets need to have a 'skirt' at the bottom, partly for an aerodynamic design and partially so that they create a seal down the barrel, allowing them to be fired.
A BB is a minuscule sphere. They usually hurt less than pellets and require less pressure to launch. Most BB's are made from Copper or painted lead.