A database of the properties for sale in a particular market. This database is maintained by an organization associated with the local Designated Brokers. The relationship between the organization and the local brokers differs from state to state and even within states, from a cooperative ownership of all, to ownership by the local chapter of the National Association of Realtors.
In addition to the database, an MLS organization provides the resources necessary for Real Estate Brokers to assist in the purchase and sale of real property. This ranges from electronic keyboxes, to contract forms, to extensive training and education.
The hallmark and defining characteristic of an MLS, however, is not the support it provides to brokers. Rather, by rule, it is cooperative. Every listing - without exception! - must offer two commissions: one to the listing agent, and one to the selling agent. And for reasons unknown (although collusion is likely the answer) the commission to the selling agent is almost always 3%.
The acronym “MLS” is used as shorthand. In Seattle (and most of Washington State) the official name is the Northwest Multiple Listing Service (or NWMLS).