OK, that's a gross oversimplification, but it really does appear to be the case that Earth's single moon was forged from the remnants of a planetary collision. Billions of years ago, Earth was sterile and toxic to any would-be life forms, well on its way to an unremarkable future. But then this cataclysmic collision occurred, drastically altering the face of the planet - as well as its angular velocity and composition. The debris ejected by this global merger coalesced into another rocky body itself, which currently hangs in a somewhat stable orbit and lights up our night sky with its cold, cratered face.
Some of the clues which lead to such an interesting conclusion are themselves rather striking. The Moon is covered in a layer of dust, but beneath that are some of the oldest rocks that have ever been found, which helps us date the collision. These rocks are also completely dry. And in some places, there is orange volcanic soil beneath the dust. What these clues tell us is that, since the oldest rocks on the Moon are about as old as some of the oldest rocks from Earth, these rocks probably cooled at about the same time. The total absence of moisture in these rocks indicates that the Moon was very hot at some point, even having volcanoes of its own.
So, at this point, the most likely explanation seems to be that Theia, this planetary body about the size of Mars, collided with the Earth and all the hot matter ejected into space from that collision coalesced into a volcanic satellite, which cooled over time into the moon of today.
Here's a bit from the History Channel on the giant impact hypothesis: