IDX (Internet Data Exchange) is how MLS listings end up on a website.
Also known as Broker Reciprocity, IDX encompasses the policies, rules, and software that allow listings from the MLS database to be displayed publicly.
Anytime you see properties on a website that came from an MLS, it was made possible through IDX.
Most real estate agents and brokers use IDX to simply display MLS listings or home search tools on their website, but as home buyers have become more Internet-savvy, IDX has evolved to encompass more. Agents today have the option to use basic home search tools provided by their MLS, or build more advanced IDX solutions; usually with the help of an IDX vendor.