A Multiple Listing Service (MLS) is an organization of real estate brokers that agree to share home listings with one another. They also agree to compensate one another if they sell a listing from another member of the MLS. If Broker A has a home for sale, they share the listing with Broker B who is also a member of the MLS. This allows other brokers to show and sell listings from fellow MLS members.
Multiple Listing Services are usually local or regional organizations, often run by the local Board of Realtors, but sometimes owned and operated by their member brokers. Most publicly-accessible real estate websites obtain their listings from MLS data feeds.