OpenStreetMap is a crowd sourced map of the world which has grown to become one of the most detailed sources of local-scale map data available. Source map data is created and maintained by thousands of volunteers from around the world, using processes similar to the maintaining of the wikipedia encyclopedia.

The most visible aspect to OSM is the online web-tile interface from https://www.openstreetmap.org/, but the maps can also be viewed, imported, or edited in many applications such as Quantum GIS, OpenLayers, ArcGIS and dedicated OSM applications.

The heart of the project is the underlying data which is open for all to edit, view, or create custom rendered maps. Fundamentally OSM’s focus is on the data, the rich maps simply fall out of this.　　

License: Creative Commons CC-By-SA
The project description quoted above is licensed by OSGeo under a Creative Commons Attribution 3.0 Unported License。