OpenStreetMap is a free, editable map of the whole world. Unlike proprietary datasets like Google Map Maker, the OpenStreetMap license allows free access to the full map dataset. This massive amount of data can be downloaded in full, but also is available in immediately-useful forms like maps and commercial services.
The main way that users participate in OpenStreetMap is by editing the map. With a free user account, you can make improvements to the map that fix issues and add data for everyone. Some users take GPS units on walks, drives, or cycling trips to record tracks that can then be imported to OpenStreetMap. Others help out by tracing roads and features they find in satellite imagery into the map.
OpenStreetMap’s funding and infrastructure is supported by the non-profit OpenStreetMap Foundation. While it doesn’t maintain ownership of the dataset or control of the project, the foundation encourages the growth, development and distribution of free geospatial data for anybody to use and share.
You can start editing the map straight away using the online ID or Potlatch editors, or download the JOSM desktop application.