CIF was developed by the International Union of Crystallography. It was adopted in 1990 as a standard file structure for the archiving and distribution of crystallographic information. The full documentation and specifications of the format can be found here: https://www.iucr.org/resources/cif/documentation#docs.
There are databases which allow you to download CIF files for various minerals.
The American Mineralogist Crystal Structure Database includes every structure published in the American Mineralogist, The Canadian Mineralogist, European Journal of Mineralogy and Physics and Chemistry of Minerals, as well as selected datasets from other journals. It is a good source for mineral CIF files
Pay attention though! The CIF files available in this database correspond to published data in scientific publications. They can be minerals of unusual compositions or unusual conditions of pressure and temperature. Sometimes, basic minerals at ambient P and T are not in the database. The reason is simple: these structures are known and no one will publish a paper in American Mineralogist to describe the structure.
CIF files can used when creating synthetic datasets with PolyXSim for testing and learning Multigrain XRD.
Specific examples: