By Listing your Property on the MLS, a MLS Listing is created, IDX forwarding is enabled, resulting in world wide exposure to all enabled web sites.

IDX | Internet Data Exchange - 

-From NAR Broker IDX Policy Statement
Internet Data Exchange ("IDX"), also referred to as "Broker Reciprocity," is the next stage in the evolution of MLS as the primary means of enhancing cooperation between REALTORSŪ to facilitate the purchase and sale of real property. IDX gives MLS Participants the tool they need to display each others listings on their Internet websites. Under IDX brokers exchange consent to display each others listings on the Internet.
IDX gives brokers and real estate agents the ability to display MLS listings on to their company and individual real estate websites. The upshot for home sellers is that when a property is listed in the MLS with one broker, it now becomes available to customers through every cooperating Realtor website including Realtor.com and websites that display property search results from Realtor.com like Yahoo.com, MSN.com and others. In fact your local, in town independent brokerage firms as well as all franchise firms websites will normally all display the same listings from the IDX program.
So, if you list on the MLS with Town and Country Apollo Properties, your home is guaranteed by the National Association of REALTORŪs (NAR) broker IDX policy to appear on all of the real estate websites that display property listing information. That's a tremendous amount of exposure for your property. Millions of home buying customers around the world now have access to your property's information through the country's most comprehensive real estate listing database.