OGC Catalog service for the web (CSW 2.0.2)
The OpenGIS® Catalogue Services Interface Standard (CAT) supports the ability to publish and search collections of descriptive information (metadata) about geospatial data, services and related resources. Providers of resources use catalogues to register metadata that conform to the provider's choice of an information model; such models include descriptions of spatial references and thematic information. Client applications can then search for geospatial data and services in very efficient ways.
A useful basic description of CSW function can be found at this OGC Tutorial.