The information comes from what Google calls maplets, small applications created by Google or third parties that allow Google Maps to be combined with any other service, such as a real estate listing or a report on the state of traffic.
Users who set up a Google My Maps account can create their own custom maps, adding as many maplets as they like. They can also draw directions on their maps, as well as link photos or videos of specific locations using services like YouTube and PicasaWeb.
The resulting maps can be kept private or made public and indexed by the Google search engine, as indicated on the My Google Maps page. It is also possible to upload Google Earth maps as a file in KML (Keyhole Markup Language) format, a technology that allows describing geographic coordinates.