MLS is an acronym for Multiple Listing Service. The multiple listing service is a forum in which participating real estate agents can list properties they have for sale. By listing properties in this forum, listing agents are able to market properties to a larger audience, namely buyer's represented by other agents that participate in the multiple listing service.

Marketing a home for sale is one of the primary jobs of a listing agent. If they do not let people know your home is for sale, it will not sell. The MLS is an effective way to introduce a new home to the market because it introduces the home to agents in your area that are not part of the listing agent's company.

By in large the MLS is one of the largest marketing tools an agent can use. It is highly recommended that the listing agent you choose has access to, and is willing list your home on the MLS.

Multiple Listing Service

In the old days this was a closely guarded "book".  Today it is quite a sophisticated application/database of  properties for sale or lease etc.  With both public and "private" (to members only) information about the property.  Today many industry participants have "access" to this information, some locally, and some have access to MLS databases nationwide.  ie:  when you go to realtor.com to search for a property in some area -- it is accessing that area's MLS information.

What information is private or public depends on the local association.  These resources today are commonly linked to other data bases such as public tax records.