.OSM File Extension
What is an OSM file?
An OSM file is a street map saved in the OpenStreetMap (OSM) format. It contains XML-formatted data in the form of "nodes" (points), "ways" (connections), and "relations" (street and object properties, such as tags).
OpenStreetMap is a global collaborative project that maintains a free, up-to-date planetary map. This map, which is named planet.osm and available for download online, shows streets, geography, landmarks, and other items of interest.
Planet.osm contains a lot of data, so it is an incredibly large file. Users who want to work with smaller sections of planet.osm's data can do so by exporting only that portion of the file from OpenStreetMap.org. For example, a user who wants to export a map of their city can zoom in on their city in OpenStreetMap.org and select Export. OpenStreetMap.org will then produce a map.osm file that contains the map shown on the user's screen. Users can also download more targeted OSM files from supporting FTP sites, which are linked from OpenStreetMap.org.
Common OSM Filenames
Planet.osm - The name of OpenStreetMap's complete street database.
Map.osm - The default name assigned to maps downloaded using OpenStreeMap.org's Export tool.
How to open an OSM file
You can open an OSM file in many mapping and GIS programs, to view, mark up, and edit the map the file contains. For example, you can open an OSM file in Java OpenStreetMap Editor (multiplatform), QGIS (multiplatform), or Merkaartor (multiplatform).