QGIS, or Quantum GIS, is an open source GIS application framework: QGIS distribution is mainly a map viewer and editor, enhanced by numerous plugins.
QGIS upstream is a GitHub repo. QGIS hosts its own poorly organized official site, which has a Redmine issue tracker and lists all QGIS plugins.
QGIS project file with extension .qgs is an XML file storing paths to data files
(default to relative paths), project projection and transformation, layers, style, composer, etc.
The QGIS project file format may not be backward compatible.
QGIS style definition file with extension .qml is the part of a QGIS project file for map rendering.
Graphical Interface:
Advanced digitizing panel; Tracing (base layer);
Spatial bookmark: save and name a viewport.
Expression:
wildcard: %, used together with LIKE;
is NULL, is not NULL;
list of values (x, y, z);
Beware that QGIS has different implementations for filtering (OGR SQL) and selection by expression.
Style:
† denotes customized shortcuts.
?, keyboard shortcuts†
F12, options (preferences)†;
C⇧p, project properties;
p, pan map†;
s, select features†;
i, identify features†;
e, edit (toggle;)†
a, add feature†;
m, move feature†;
n, node editor†;
t, tracing (toggle);
-, zoom out†;
+, zoom in by rectangle†;
1, zoom to data (full);
2, zoom to layer;
3, zoom to selection;
8, previous viewport;
9, next viewport;
0, zoom to native resolution;
PgUp, zoom in;
PgDn, zoom out;
Cb, add bookmark;
C⇧b, show bookmarks;
F5, refresh;
Ch, hide selected layers†;
C⇧h, hide all layers;
Cu, show selected layers†;
C⇧u, show all layers;
Ca, select all features in selected layers;
C⇧a, deselect all features;
Ce, select by expression;
Cf, filter by expression;
Ct, attribute table†;
Ci, insert/update attribute (field calculator)†;
C⇧m, measure line;
C⇧j, measure area;
Cr, raster calculator;
CMp, Python console;
Processing:
CMc, options;
CMd, commander;
CMh, history;
CMt, toolbox;
QConsolidate plugin transfers data files to a single directory and rewrites the project file.
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 2.14 Documentation on GRASS Integration
GRASS allows all geometry types (point, line and polygon) in one vector map, but QGIS requires a layer to have a specific type.
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.