The database behind Rebrickable contains a catalog of LEGO Sets, Parts, Colors and various relationships. This LEGO catalog has been put together over many years with the help of an amazing volunteer admin team, with submissions from thousands of Rebrickable members.
A simplified version of this database is periodically extracted and is freely available for download as separate CSV files at the Downloads page. This is a valuable source of information for research purposes or to kick-start your own personal LEGO project.
If you are familiar with databases, I think the above schema is fairly self-explanatory. However, one point of confusion can be how the inventories work. A LEGO Set can consist of one or more Inventories (e.g. 8088-1) We might have multiple Inventories if the Parts in the Set were changed during it's production period, or perhaps to cater for different geographic regions.