***According to Wikipedia.org***
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.
Site users generally gain the benefit of anonymous property searching and review. IDX sites usually provide less detailed information than the full Realtor Multiple Listing Service, limiting the data to that which is deemed publicly accessible. The benefit to realtors is that users of their IDX web site can search freely, establish some confidence, and qualify themselves before contacting the realtor.
Certain rules apply to the real estate companies' ability to display each detail about a property. These "display rules" are set by the Multiple Listing Service organization, which generally forms its policy around the recommendations of the National Association of Realtors.
IDX policy is nearing its replacement by a new Internet Listing Display policy being formed by the National Association of Realtors.