QGIS, or Quantum GIS, is an open source GIS application framework. In is basic state, QGIS is mainly a viewer and map builder, which can be enhanced through numerous plugins.
QGIS hosts its (poorly organized) official site and a Redmine issue tracker. QGIS Upstream is on GitHub. All QGIS plugins are listed on it official plugins site.
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%, used together with LIKE.QGIS is much less powerful than GRASS GIS, you may read this discussion about the differences between QGIS and GRASS.
QGIS ships with a GRASS plugin, acting as a more friendly interface of GRASS.
QGIS OSM support is in menu (Vector > OpenStreetMap), which can download and import OSM files by geometry type and with selected tags via a SpatiaLite database connection. OSM's 64-bit identifier is supported in QGIS.
QuickOSM plugin can send Overpass queries from QGIS to get OSM data.