Single ball shells consist of a single spherical-shaped shell that produces a burst of color and sound in all directions. Multi-ball shells, on the other hand, consist of a single shell with two or more spherical-shaped balls that can be launched together from a single tube to produce a more dramatic and varied display of color and sound. This means that with each shot, a multi-ball shell can produce multiple bursts of color and sound, while a single ball shell will only produce one. Multi-ball shells are a popular choice for those looking to create a more dynamic and varied fireworks display.