RETS

The "Real Estate Transaction Standard (RETS) is an open standard for exchanging real estate transaction information. The standard consists of a transaction specification and a standard Extensible Markup Language (XML) Document Type Definition (DTD).  RETS is being implemented by many real estate industry leaders in their next generation of real estate information systems." 

The Real Estate Transaction Standard Protocol Specification documents the exchange of real estate property information, with the intent of eventually describing all interchangeable aspects of a real estate transaction. It defines a standard interface by which a client program may communicate with an property data server. The specification defines a protocol for implementing transactions, and incorporates an XML vocabulary for general purpose interchange. The specification also provides for a compressed data interchange format and specification to allow interchange of machine-interpretable property information. This specification works with the Real Estate Transaction DTD that defines the general structure of to be used in transferring information from the server.

You can learn more about RETS at www.rets-wg.org.  The web site is maintained by the RETS Working Group, which is the organization responsible for publishing, maintaining and enhancing the standard. The RETS Working Group maintains three mailing lists to which interested developers and users can subscribe for information and discussion about RETS.