OpenStreetMap (OSM for short) is a global project to create a free and freely accessible map of the world. Most importantly, anyone can edit it, so it is called a cartographic equivalent of the Wikipedia.
Any data the project contains is shared on terms of the Open Database License (ODbL), so it can be reused, among others, in GPS navigation systems, which makes it a free alternative to expensive and not necessarily better commercially available maps. The project, started in 2004, is now maintained by over 600 thousand mappers (as the OSM editors call themselves), and all the objects sent to the database take over 200 gigabytes.
How does it work?
Map is being created in a few ways. The oldest one is gathering traces with a GPS receiver and then, after uploading them to a computer, drawing streets along these traces.
The other way is to use available satellite imagery that may be used to draw not only roads, but also buildings or unavailable places. This way one can create a map of regions he didn’t even see. Thus was created e.g.a map of Haiti capital, that was priceless for rescue actions after 2010 earthquake, made mainly by the mappers from Europe and USA.
It has to be noted, that due to rigorous approach to copyright, it is forbidden to copy from other maps or importing the data that has not been released for us.