The STAC Specification consists of 4 semi-independent specifications. Each can be used alone, but they work best in concert with one another.
STAC Item is the core atomic unit, representing a single spatiotemporal asset as a GeoJSON feature plus datetime and links.
STAC Catalog is a simple, flexible JSON file of links that provides a structure to organize and browse STAC Items. A series of best practices helps make recommendations for creating real world STAC Catalogs.
STAC Collection is an extension of the STAC Catalog with additional information such as the extents, license, keywords, providers, etc that describe STAC Items that fall within the Collection.
STAC API provides a RESTful endpoint that enables search of STAC Items, specified in OpenAPI, following OGC's WFS 3.