Source Criticism was a study that took off in the 19th century and the basic gist of this approach viewed the Pentateuch (first five books of the Old Testament / Hebrew Bible) as a compilation of various sources that were stitched together over a vast period of time.
The Documentary Hypothesis was formulated by Julius Wellhausen that stated that there were four sources (J, E, P, D). He argued that the combination of these four sources created the Pentateuch. He argued the following:
J or the Jehovah source was composed around 850 B.C. in Judah and used the divine name of Yahweh. It also used a simple style in terms of grammar and God was presented in anthropomorphic ways.
E or the Elohim source was created around 750 B.C. to be a corrective to the J document. For example, it does not like anthropomorphic language, and after some time (around 720s) J and E were supposed to have been combined.
D or the document written around 650 to promote the purity of Judah’s worship. And the name of God was often a combination of Yahweh and Elohim. The JED sources were supposedly combined into one document sometime after 587.
The P document was written sometime during the middle of the fifth century to address the lack of priestly concerns. Wellhausen believed that the sacrificial material dated to this revision. Around 400 B.C. The JEDP sources were combined to create one source.