Data providers use the Open Archives Initiative Protocol for Metadata Harvesting (OAI-PMH) to expose their metadata for harvesting, i.e. if you’re a data provider then YOU provide the metadata for OTHER sites.
The following topics give examples of the requests and responses for OAI-PMH.
For more information about the Open Archives Initiative Protocol for Metadata Harvesting go to http://www.openarchives.org/OAI/openarchivesprotocol.html.
For an example of how one site exposes their records through OAI-PMH you can go to the Library of Congress site at http://memory.loc.gov/cgi-bin/oai2_0?verb=ListRecords&metadataPrefix=marc21.