Simply put, a multiple listing service is a database that allows us agents to share listings with each other, as well as view market information. You can almost think of it as a bare-bones social network, where agents can post what they want, but comment on nothing. Each region has its own MLS. For example, on the MLS in my area, I can see listings in the Greater Boston area, but I cannot see listings in California. If I wanted to see listings there, I would either have to rely on a third-party website (Like Zillow), or call a colleague on the West Coast.