Multiple Listing Service (MLS) (also Multiple Listing System) is a real estate listing service that combines the listings for all available properties in an area, except For-Sale-By-Owner (FSBO) properties, in one directory or database. The information is available online at various MLS websites (for example http://www.mls.ca/).
In general, MLS access is restricted to licensed real estate agents. Those agents pay a fee to view the listing database.
The general public only gains access to a portion of the overall listing database via a portal or other website (as in the example above). This portal only displays content that has been approved for display according to Virtual Office Website VOW or Internet Data Exchange IDX rules.
Currently, in the United States there is no one central MLS. MLS's are regional entities that can span several states, or just a single county. They are typically run by the local association of realtors.