The core of a SpatioTemporal Asset Catalog (STAC) is a set of JSON fields defined by the STAC Item spec. These fields define an Item - the atomic units that contain metadata for search as plus links to the actual assets that they represent. Their main function is as the leaf nodes of a Catalog. See the overview document for more information on how all the pieces fit together.
Item Specification: The main definition of the STAC Item specification is in item-spec.md. It includes an overview and an in-depth explanation of the fields.
Common Metadata: A set of commonly used metadata fields for STAC Items is listed in common-metadata.md.
Examples: For samples of how STAC Items can be implemented the examples/ folder contains a number of real world examples. See the readme for additional discussion of the examples.
Schemas: The schemas to validate the core Item definitions are found in the json-schema/ folder. The item.json validates items overall and the additional schemas validate the various groups of Common Metadata.