Capture Theory: The moon was captured by the gravitational field of the Earth, but this would not explain why the moon orbits the Earth in the same plane as the Earth orbits the sun. Many moons of other planets appear to be captured satellites, but they are very small and have very odd orbits.
Twin Theory: The moon formed alongside the Earth during its early development. However, one would then expect the Earth and moon to be made of the same material, which they are not.
Impact Theory: The early Earth was impacted by a large object, perhaps the size of Mercury. It then jettisoned a large amount of exterior material into space. This theory explains why the moon is made of mostly lighter material than the Earth, and why it is so large. The impact theory is now the most widely accepted