QGIS has a plugin infrastructure. You can add a lot of new functionality by writing your own plugins. These plugins can either be written in C++ or in Python.
If you want to contribute to QGIS Project with your plugin, it’s highly recommended to read Developers guide for QGIS
Inside the QGIS folder:
cd src/plugins/ python plugin_builder.py
Now follow the instructions in the screen wizard to create a folder for your plugin. After the required input is given, a folder for the plugin is created. Read the README inside the plugin folder to create a plugin.
Building QGIS now will also build your plugin. After starting QGIS enable the plugin to see it and show its dialog.
QGIS has a lot to offer for python developers too.
QGIS has python bindings so you can automate tasks in QGIS via python.
Interested in python plugin development, easiest way to start is using the Plugin Builder plugin in QGIS. Search for ‘Plugin Builder’ in the plugin manager of QGIS, or read up here: http://plugins.qgis.org/plugins/pluginbuilder/
Looking for examples of python plugins, see http://plugins.qgis.org
You can find the QGIS-iface which you can use via python here:
http://qgis.org/api/classQgisInterface.html (for QGIS testing)
http://qgis.org/api/2.0/classQgisInterface.html (for QGIS 2.0)
http://qgis.org/api/1.8/classQgisInterface.html (for QGIS 1.8)