QGIS is a modular and extensible open source geographic information system. It has a Python API that allows developers to build plugins that interact with QGIS functionality and libraries. Plugins can be built with C++ or Python and interface with QGIS through the QgisInterface class. Example plugins include InaSAFE for hazard impact scenarios and Crayfish for visualizing flood modeling results. Developers can also create standalone applications using the QGIS libraries for tasks like automating GIS processes, building custom editing tools, or developing specialized mapping applications.