A STAC Catalog is a top-level object that logically groups other Catalog, Collection, and Item objects. A Catalog contains an array of Link objects to these other objects and can include additional metadata to describe the objects contained therein. It is defined in full in the STAC Catalog Specification.
For more information on how the parts of STAC fit together see the overview document.
A Catalog is typically the "entry point" into a STAC object hierarchy. For example, the root endpoint ("landing page") of a STAC API implementation is a Catalog. For many static STAC catalogs (e.g., those defined only by a set of files on disk or in a cloud object store), the root URL points to a Catalog that acts as the starting point to traverse the entire catalog of Catalog, Collection, and Item objects.
While STAC Catalogs mostly describe a structure of links and Items, a key related specification is the STAC Collection Specification, which contains fields that further describe the group of Items in a Catalog.
A STAC Catalog requires a subset of the fields required by a Collection.
These are distinguished from one another by the type field, which will have the value Catalog or Collection.
This means that a Collection can be changed to a Catalog simply by changing this type field.
The parent-child relationships among Catalogs and Collections are for objects of these types,
as there is no subtyping relationship between the Collection and Catalog types, even through they share field names.
Catalogs are designed so that a simple file server on the web or object store like Amazon S3 can store JSON that defines a
full Catalog. More dynamic services can also return a Catalog structure, and the STAC API
specification contains an OpenAPI definition of the standard way to do this, at the / endpoint.
Specification: The main Catalog specification is in catalog-spec.md. It includes in depth explanation of the structures and fields.
Schemas: The schemas to validate the core Catalog definition are found in the json-schema/ folder. The primary one is catalog.json.
The Catalog specification is maturing, but it is still relatively early days. The core of Catalog has been defined very narrowly, to just describe a structure that can be followed by people or machines, so most additional functionality will be defined in additional specifications and extensions. The only anticipated changes to the core of Catalog are to add in additional extension mechanisms for others to use.