Data catalogs act as an index of datasets, using metadata to describe the content and location of data. As opposed to a repository that stores the data in one place, a data catalog describes data wherever it is stored. Data catalogs may also describe restricted data, outlining how a person can apply for access to data that is restricted due to ethical or legal reasons.
NYU Data Catalog describes datasets, their location, and any access restrictions: https://datacatalog.med.nyu.edu
NLM Data Catalog is a work in progress. When completed it will describe data across various NIH repositories. Information about the metadata schema and progress on the project is available online: https://app.smartsheetgov.com/b/publish?EQBCT=0e99b413c0a145be926850e250cee45c
Health Data Gateway is a data catalog for biomedical data generated in the United Kingdom: https://www.healthdatagateway.org