Web services are services that enable a user to search for spatial data resources based on the content of the corresponding metadata. The metadata inform about the properties of the existing spatial data and services. A cross-border search and research can only be useful if the metadata is made available in a standardized manner.
The underlying standards are
ISO 19115 - for the description of the spatial data
ISO 19119 - for the description of the spatial data services
The standard for the realization of the discovery services is
OGC CSW Catalog Service Implementation Specification in version 2.0.2.