AKA Documentary Hypothesis

A theory stating that the Torah, the five books of Moses, was compiled over time, primarily from the works of four people (or more likely, groups of people) working off of oral tradition, rather than being the work of God or even necessarily divinely inspired. This provides a very simple explanation for the many contradicions found in the Torah, as each piece of the contradiction can usually be attributed to a different author. Perhaps both were left in because no one could be sure if one was more correct than the other, or perhaps it was simple editorial oversight. Either way, it's a pretty cool theory.

The four people/groups have different writing styles, identifiable throughout the Torah and have been given the highly inventive yet functional names J, E, P, and D. Some also name an R, the Redactor, who editted the separate works and put them all together, forming the Torah we enjoy today.

The four others, in a nutshell:

Note: J and E are thought to be the earlier of the four sources, and at some point their works were likely merged. Because of this, it's sometimes hard to tell them apart.