The Datalog is a storage of accumulated data that you can pick up in the game and that the game records for you in the actual Datalog item. A copy of the data and entries is included below for reference though you can access it in the game as well.

The content featured in this section, specifically the text from the Final Fantasy XIII Datalog, is transcribed from the game and is recorded for informational purposes. The information is owned by Square Enix Co., Ltd.