MLS stands for Multiple Listing Service and it is a real estate listing service that gathers information about homes listed for sale by members of the local board of Realtors.  Since this database is essentially a compilation of listings held by various real estate brokerages, it does not include homes that are "For Sale by Owner", this is homes listed for sale without a Realtor. Your local real estate agent has access to the complete MLS database, which is updated on a regular basis and includes details about each home listed.

Also, through the MLS your real estate agent has access to prices for homes sold through the MLS.  This information is used to help agents determine the fair market value of any home you are considering purchasing.