The MLS is an acronym for Multiple Listing Service which is a resource Realtors use to help market and sell active listings. The MLS# is a simple way of tracking a particular home. Just like an APN (Assessor's Parcel Number) used by your country's tax assessor, this allows particular agents to identify specific homes without question of like addresses.

The MLS is a service that must be subscribed to, however many agent subscribers have made it available to their clients in hopes of searching current listings. If you have access to the MLS, you can conduct an MLS# search, which will take you to the specific home the MLS# refers to.

MLS simply means multiple listing services.  Think of it like the database for all homes that are for sale in your area.  Each property is assigned an MLS number which is another way to identify the property in this huge database instead of using just the address. MLS also keeps records on homes that are pending or in a contract process heading towards sold and records for the sold properties.  MLS is a very useful tool for Realtors and sellers who can find all properties currently on the market and assist buyers in viewing these homes.  Every state can have more than one MLS system depending upon Realtor associations and the contracts they may have with a database provider for MLS services.  You may find a main city in your state uses one MLS service while another city a few miles over is using another service provider.  Often the realtor associations will co-op for mls access and keys so realtors in one area but close to other areas using another mls provider can still sell a home in the other area. 

Hope this helps answer your question.

The MLS # is a number assigned by the Multiple Listing Service(MLS) to a listing when it is being sold by a Realtor (a Realtor is a person who is licensed by the state they are selling in AND who is also a member of the National Association of Realtors).  It is a number you can you use to look up a property using the various free mls search tools offered by most real estate companies on the internet to find a particular home in that area's local mls. It is simply a way to look up the details of the particular property as posted by the agent who is selling the property on behalf of the owner.