"An Internet Data Exchange (IDX) is a real estate property search site which allows the public to conduct searches of approved Multiple Listing Service properties in a certain area." Learn more here.
Like RETS feeds, IDX feeds are unique and complex methods to syndicate and display Multiple Listing Service data. These feeds can be difficult and time consuming to integrate with your website. If you are creating an application to display MLS data with IDX, you will need an experienced IDX developer.
I have worked with several IDX services and can help you get your MLS listings online.
It's never cheaper and never faster to do this twice (or more!).