QGIS is free, open-source desktop GIS software. It runs on Windows, Mac, and Linux.
QGIS is a full-fledged desktop GIS. It reads an writes nearly all geospatial data formats, enables amazingly dynamic cartography, and provides a robust set of processing and analytical tools. Additional functionality is provided by over 300 plugins, which can be installed with just a few clicks. For more details, see the QGIS website.
QGIS is developed by contributors from around the world who are constantly adding new features and improving the program. New versions of QGIS are released every four months. The changelog is a great way of learning about the latest features.
QGIS is free and open source software, which means you can download a copy for your own computer and be up and running within just a few minutes. Installers for Windows, Mac, Linux are available from the official QGIS website
Windows users have the option of installing from the OSGeo4W Network Installer or the QGIS Standalone Installer:
Standalone installer -- recommended for new users
OSGeo4W installer -- for more advanced users, it provides a way to upgrade QGIS to newer versions as they become available, and also provides access to other open source geospatial software
Please watch this "how to install" video