MLS - multiple listing service - database and software of real estate listings used to distribute information amongst brokers for an opportunity to create cooperation with another broker who is representing a buyer. MLS systems contain hundreds of different fields of information about the features of a property, however only few of them will be displayed to the public.
There are many MLS across the world and the US. The largest one is the California Regional Multiple Listing Service (CRMLS). There are also big MLS systems in Maryland, Phoenix and NY.