OpenStreetMap is built by a community of mappers that contribute and maintain data about roads, trails, cafes, railway stations, and much more, all over the world.
OpenStreetMap is a free, editable map of the world that anyone can add to, sometimes described as the Wikipedia of maps.
Those mapping, who co-ordinate and help each other, break the imagery into “tiles” and literally trace roads, buildings and highways onto the map.
The work of new mappers is validated by more experienced mappers to ensure accuracy. Just like a wiki.
Here is the resource http://www.openstreetmap.org/about
Saira Asher from the BBC has written on how How ‘crisis mapping’ is helping relief efforts in Nepal using OpenStreetMap.