The Real Estate Transaction Standard is a common language spoken by systems that handle real estate information, such as multiple listing services. A common language enables computers like the one on your desk to receive information from many different real estate systems or MLSs without being specially "trained" to understand the information from each.
Standards like RETS exist in many different fields. Sometimes, the standard simply adopts one of many pre-existing languages that everyone agrees to use. For example, air traffic controllers at international airports all speak English, no matter what their native language, so pilots are guaranteed that they need learn only one language to fly anywhere in the world.
RETS, like many computer standards, is a language that was built for a specific purpose, but the goal is the same: to have all computers that deal with real estate information "speak" the same language, so that you can use the same desktop computer program with any MLS that has adopted RETS.
For software developers and for providers of services like IDX sites, RETS means having to write programs to use only one language, the common language of RETS, in order to work with many different MLS systems. This means lower costs, more products, more competition among vendors, and faster implementations of new systems, all of which directly benefit people who work with real estate information as a living.
For a more comprehensive overview please review the documents below.
NAR MLS/Client Data Standards Whitepaper (“Summary”)
RETS Update: What you need to know
RESO Application Interoperability Best Practices Guide (open for comment)
RETS 1x tools libRETS and ezRETS support sites.