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 prefix Qgs is the Qt namespace registered by Gary Sherman, the QGIS creator, so Q stands for Qt and gs stands for Gary Sherman.
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")