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.
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.