A "Multiple Listing System" (MLS) is a database system allowing real estate brokers with a listing contract to "list" the property, making the information available to other real estate agents who might have a potential buyer for the property.
The majority of MLS Services restrict access to licensed real estate professionals. In most cases a person selling their home themselves in a for sale by owner situaiton can not directly list their property.
However, with the growth of the internet as a means of locating properties some brokers are offering to list properties for owner sellers for a flat fee. Though this is a new trend it has rapidly become one for the fastest growing areas in real estate.