Jupiter completes one full rotation on its axis every 9.9 hours. It completes a full rotation faster than any other planet in the Solar System.
Jupiter is the largest plant in the solar system, but is also composed completely of gases so it is not as heavy as the terrestrial planets. This allows it to spin faster on its axis because it has a much lighter mass.
Because Jupiter is so large, and spins at such a rapid rate, the equator of the planet bulges out. This makes Jupiter appear to be more of an oblong shape rather than a perfect circle.