The multiple listing service (MLS) is an online database of homes being offered for sale by participating real estate agents to other participating real estate agents. (It's like Zillow® for real estate agents and their clients.)
It is also the primary data source for the public real estate portals like Zillow®, realtor.com® and Homes.com®.
Real Estate agencies like Redfin®, Century 21®, RE/MAX®, Coldwell Banker® and others also use this data for their public websites and the websites of their agents.
When a home is listed in the MLS, it is syndicated to thousands of websites including all the real estate portals, real estate brokerages and real estate agents for massive exposure. It is estimated that exposure from the MLS is about 400% greater than that of Zillow alone.