Standalone installer: install GRASS GIS with the required support packages.
OSGeo4W is an installer for a broad set of open source geospatial software packages including GRASS GIS as well as many other packages (QGIS, GDAL/OGR, and more).
The following install modes are offered (see also the dedicated wiki page for further details):
Run the downloaded installer as administrator and select the grass (stable) or grass-daily (development) packages to install. All needed dependencies (e.g. GDAL, common python packages) will be automatically downloaded.
All packages can be individually selected and additional useful packages (e.g. python-pandas, msys) are available, too.
Installation can also be done through command line, as shown below:
osgeo4w-setup-x86_64.exe -q -k -P grass -s http://download.osgeo.org/osgeo4w/x86_64/
In order to have GRASS GIS support (also in QGIS-Processing) as well as the “With GRASS” shortcut you need to install the “qgis-grass-plugin*” packages.