A multiple listing service (MLS) is a marketing database set up  cooperating real estate brokers to provide accurate and structured data about properties. 
It's also a mechanism for listing brokers to offer compensation to buyer brokers who bring buyers for their listed properties.

MLSs are private databases that are created, maintained and paid for by real estate professionals to help their clients buy and sell property. Access to information from MLS listings is provided to the public free-of-charge by participating brokers. 
Data that is not publicly accessible includes information that would endanger sellers' privacy or safety, such as seller contact information and times the home is vacant for showings.

MLSs are a powerful force for competition. They level the playing field so that the smallest brokerage can also compete with the biggest firms. Buyers and sellers can work with the professional of their choice, confident that they have access to the largest pool of properties for sale in the marketplace.

Accuracy of Information

• MLS listings databases are considered the most accurate as far as property details on the Internet or elsewhere. New listings must be entered within a specified time period and with accurate data.Fines are generated if a member doesn't enter the full listing data by the deadline. Generally, this is 24 to 48 hours after the listing agreement is signed.
  

• From size of property to land size to directions and location, the MLS association wants to maintain a high level of accuracy in the information that's entered. The quality and number of photos included are often mandated as well, although the exact criteria of this requirement can vary from time to time.