So what exactly is the MLS? It is not a website. It would be incorrect to call it a database of properties. It is more like a trade association. Members of the MLS are real estate brokers and agents and the main function of the MLS is to provide a means for brokers to cooperate in the selling and buying of homes.
These organizations are currently local although there is a trend towards regional and statewide MLS databases and organizations. There are around 900 in the country. There is no "National MLS" as some websites proclaim. AND mls.com is not the MLS.
The main way these brokers share listing information is through the MLS database. Access to this database is private and for "members only". Although limited access to MLS database information is allowed through various mechanisms. MLS data can be shared with clients, display other brokers properties on their websites, and other licensed parties like appraisers.
The MLS really exists to allow brokers to cooperate with each other and get paid when clients that they represent buy or sell homes.
A for sale by owner MLS listing is really misnomer. Once a for sale by owner is listed on the MLS it usually means that they are being represented by a broker in some capacity. Many times flat fee listing companies use exclusive agency listing agreements. This means that the sellers retain the right to sell their home on their own without a broker.