What RETS means to a developer
With RETS you are able to create one interface to access any RETS data source. Essentially, your application would be able to scale to include any data set without creating custom code to access and retrieve the data. This also means your application can be built independent of a specific Real Estate Market/Demographic, therefore giving you a wider audience that you can work with.
Getting Started
The best way to get started with RETS is to read the RETS documents, and then get access to a RETS Server and RETS Client. You may be able to get access to a RETS Server from the MLS which you or your client are working with. Your MLS may also provide access to a RETS Client.
Additionally, you may want to download RETS Servers, clients, and other RETS tools, many of which are open source and freely available. These, which include reference implementations and demos for RETS 1.5, 1.7 and RETS2, are available from the Resources page. The available code is in Java (client and server), Visual Basic (client), PHP (client), and several other languages. Use these code examples in concert with the RETS documents, which govern what it means to be RETS-compliant.
Certification/Compliance
Once your product is built and working, you can become RETS-certified. Certification allows you to use the RETS logo on your product and in your marketing materials, and places your product in the list of compliant servers or clients on the RETS web site. Complete instructions for becoming certified as RETS-compliant, including the necessary license agreement and other documents, are on the Compliance web page.
Community Support
There is an active RETS developer community mailing list. Feel free to ask questions at any level; the list's subscribers range from those just getting started to some of those who authored the specification, so no question is too simple or too technical. There are also vendors who can help you with actual coding tasks, providing anything from libraries to full product development.