QGIS is a cross-platform desktop geographic information system that allows users to "create, edit, visualize, analyze, and publish geospatial information." In addition to functioning as a complete, standalone GIS system, QGIS can also be integrated with other open source GIS software, including PostGIS, GRASS, and MapServer. Plugins that extend QGIS can be written in the Python or C++ programming languages. Users who are less familiar with GIS concepts may appreciate the guide "A gentle introduction in GIS," located in the Documentation section on the QGIS site. That section additionally features the "QGIS User guide," which provides a technical reference to the software. Also available here, the "QGIS Training Manual" contains a series of 19 modules that demonstrate the features of the software in the style of an academic course, complete with self-test activities and answer keys. QGIS is free software, licensed under the GNU GPL. Installers for Windows, macOS, and Linux can be located on the Download section of the site.