The iface class used in the preceding snippets is important in every PyQGIS code; it is used to access most graphical QGIS components, from displayed layers to the toolbar buttons.
Note
The iface class is a Python wrapper for the C++ class, QgisInterface, which is documented at http://qgis.org/api/classQgisInterface.html. Most QGIS classes have a Qgs prefix. Some special classes can have the Qgis or QGis prefixes. The Qgs is the Qt namespace registered by Gary Sherman, the QGIS creator, so Qt (Q) and Gary Sherman (gs).
The most common use of the iface class is to get a reference of the canvas where maps are displayed:
canvas = iface.mapCanvas()The class can also be used as a shortcut to load raster or vector layers; for example, loading the raster, path/to/my/raster.tif, and naming it myraster in the legend panel. This can be done by typing the following command:
iface.addRasterLayer("path/to/my/raster.tif", "myraster")